The Influence of Lung Injury due to Mechanical Ventilation on the Initiation of Ecmo

Before the entry criteria for extracorporeal membrane oxygenation (ECMO) are met, newborns may require aggressive mechanical ventilation which may result in lung injury. The question arises whether the presence of a pneumothorax in these infants plays a role in the prognosis. Of the 21 newborns transferred to our hospital for ECMO, 8 were treated with ECMO. 9 of the 21 newborns developed a pneumothorax with conventional ventilation and 6 of these 9 newborns subsequently required ECMO. Infants who developed a pneumothorax but did not meet ECMO criteria and remained in the oxygenation index (01) range between 25 and 40 for more than 2 days had a poorer prognosis. If adequate oxygenation cannot be attained with acceptable mechanical ventilation and a more aggressive ventilation results in a pneumothorax, ECMO should be considered even if the oxygenation index is below 40.

A gene coding for tRNA is located near 5' terminus of 16S rRNA gene in Zea mays chloroplast genome

A region of 635 base pairs preceding a gene for 16S rRNA in the Zea mays chloroplast genome has been mapped and its sequence has been determined. Screening for structural elements common to tRNAs reveals a gene coding for tRNA(Val) (GU(U) (C)) positioned 303 base pairs proximal to the 5' end of the 16S rRNA gene. Both the tRNA(Val) and the 16S rRNA are coded in the same DNA strand. The tRNA nucleotide sequence predicted from the DNA sequence meets all structural characteristics common to tRNA primary and secondary structures. In a quantitative comparison with primary structures of the 14 known tRNA(Val) species the chloroplast isoaceptor shows much higher homology with that from prokaryotic than that from eukaryotic species. Regions that Escherichia coli RNA polymerase protects from nuclease attack are observed 25 and 100 base pairs upstream of the tRNA(Val) gene and 105 base pairs upstream of the 16S rRNA gene. Within these regions are short sequences that are very similar to those in the -35 region of E. coli rrn and that may therefore represent all or parts of transcription initiation signals of the respective genes.

[Gaussian distribution of intelligence in VLBW preterm infants at age 5: very low correlation with very low birth weight]

BACKGROUND

Preterm infants with very low birth weight < 1500 g (VLBW) have a higher risk of developmental disorders. In addition to the common estimation of the mean intelligence values, we studied the distribution of intelligence at preschool age in VLBW infants and the risk factors influencing this distribution.

PATIENTS AND METHODS

A prospective cohort study of 277 VLBW infants < 32 weeks born in 1991-1995 and treated according to a standardized regimen in one Perinatal Center was carried out, including measurement of intelligence (Kaufman-Assessment Battery for Children) at age 5. Statistical methods employed were: explorative data analysis, correlation, chi (2)- and t-tests; the tested variables were: small for gestational age (< third percentile), perinatal acidemia (umbilical arterial pH < 7.10), perinatal hypoxia (BE < - 10), hypothermia (< 36 degrees C), hypoglycemia after the first day of life (< 30 mg / dL), bronchopulmonary dysplasia (FiO (2) > 0.21 > or = 36 weeks), intraventricular hemorrhage, ventricular dilation, periventricular leukomalacia, seizures, abnormal acoustic evoked potentials, and hyperexcitability at discharge.

RESULTS

The distribution of intelligence in 137 VLBW infants < 32 weeks (60 % follow-up rate) was similar to a symmetrical Gaussian bell curve. The intelligence increased very slightly with birth weight (Pearson correlation: 0.172; p = 0.045) and was significantly lower in children with hypoglycemia after the first day of life (- 13.35; 95 % confidence interval: - 20.08 to - 6.63; p = 0.002), hyperexcitability at discharge (- 16.28; 95 % confidence interval: - 25.26 to - 7.31; p = 0.005), and bronchopulmonary dysplasia (- 7.00; 95 % confidence interval - 11.71 to - 2.29; p = 0.039).

CONCLUSIONS

At preschool age, the intelligence of VLBW infants is normally distributed and correlates only slightly with the very low birth weight. Hypoglycemia after the first day of life and bronchopulmonary dysplasia are risk factors for lower intelligence. Hyperexcitability at discharge seemed to represent a promising prognostic factor for a later intelligence reduction.

Focal congenital alveolar proteinosis associated with abnormal surfactant protein B messenger RNA

Two siblings presented with typical clinical features of congenital pulmonary alveolar proteinosis (PAP). Necropsy of one sibling revealed scattered foci of the diagnostic histologic changes in the lung tissue. In contrast to infantile and adult PAP, focal distribution is uncommon in congenital PAP. Defective expression of the granulocyte-macrophage colony-stimulating factor receptor was ruled out. The surfactant protein B (SP-B) content in the lung tissue of the autopsied patient was low, and a deletion in the SP-B messenger RNA was detected. We speculate that the PAP in our patients was related to the reduced quantity and/or to the altered quality of SP-B.

Leukocyte and endothelial activation in a laboratory model of extracorporeal membrane oxygenation (ECMO)

An inflammatory response and a capillary leak syndrome frequently develop during the treatment of neonatal respiratory failure by extracorporeal membrane oxygenation (ECMO). The present study was performed to investigate leukocyte activation and endothelial cell dysfunction that are associated with prolonged contact of blood components with synthetic surfaces. Laboratory ECMO was performed with fresh human blood at 37 degrees C for 8 h (n = 6). Leukocyte activation was measured by L-selectin (CD62L) and CD18 integrin surface expression and by neutrophil-derived elastase release. To monitor endothelial activation, endothelial cell ICAM-1 (CD54) expression was measured in cultured endothelial cells from human umbilical veins (HUVEC) after incubation with plasma from the ECMO experiments. CD18 integrin expression was found significantly up-regulated on polymorphonuclear neutrophils and monocytes after 2-4 h of laboratory ECMO. L-selectin was reduced on both cell types during the total duration of the experiments. Soluble L-selectin (sCD62L) and total and differential leukocyte counts remained unchanged during the experiment. Neutrophil-derived elastase content was maximal after 8 h of ECMO. Plasma from the ECMO experiments did not induce ICAM-1 expression of cultured HUVEC. We conclude that prolonged contact with synthetic surfaces during ECMO activates phagocytes, which may contribute to the inflammatory response seen in ECMO-treated patients. Activated phagocytes do not accumulate in the extracorporeal system nor release humoral factors inducing ICAM-1 expression on endothelial cells.

Do we need new indications for ECMO in neonates pretreated with high-frequency ventilation and/or inhaled nitric oxide?

OBJECTIVE

High-frequency ventilation (HFV) and/or inhaled nitric oxide (iNO) has reduced ECMO in neonates. But, frequently, improvement with HFV/iNO is temporary and only prolongs lung injury without preventing ECMO. We tried to identify a threshold oxygenation index (OI) that predicts temporary or persistent improvement with HFV/iNO in neonatal ECMO candidates as early as possible.

DESIGN

Cohort study of all neonates with OI > 40 during intermittent positive pressure ventilation between 1992 and 1997. The first treatment was HFV; at an OI > 40 during HFV, iNO was added; at an OI > 40 during HFV+iNO, ECMO was initiated. Temporary improvement was defined as secondary need for ECMO or fatal chronic lung disease without ECMO.

SETTING

University hospital level III neonatal intensive care unit.

MAIN RESULTS

Ten of the 34 neonates studied rapidly required ECMO despite HFV/iNO. Eleven neonates temporarily improved for 1-10 days before the OI was again > 40. Nine received ECMO, two were denied ECMO after mechanical ventilation > 14 days and died of chronic lung disease. Thirteen neonates persistently improved with HFV/iNO without ECMO. The OI before, at 24 h or 48 h of HFV/iNO did not predict temporary or persistent improvement. However, after 72 h of HFV/iNO, neonates with persistent improvement had lower OIs than those with temporary improvement [median OI 16 (4-24) vs 31 (20-40); P = 0.0004]. In all neonates with an OI > or = 25 after 72 h, HFV/iNO eventually failed (positive predictive value 100%, sensitivity 91 %, specificity 100%, positive likelihood ratio 91).

CONCLUSION

For neonates pretreated with HFV/iNO, an OI > 40 is an inadequate ECMO indication. Based on our data we hypothesize that an OI > or = 25 after 72 h of HFV/ iNO is a better ECMO indication that avoids prolonged barotrauma.

Postnatal changes of extracellular volume, atrial natriuretic factor, and diuresis in a randomized controlled trial of high-frequency oscillatory ventilation versus intermittent positive-pressure ventilation in premature infants <30 weeks gestation

OBJECTIVES

High-frequency oscillatory ventilation (HFOV) with a high lung volume strategy is an experimental mode of ventilating preterm infants aimed at achieving maximal alveolar recruitment Higher mean airway pressures are used during HFOV than during intermittent positive-pressure ventilation (IPPV), and the intrathoracic volume increase is relatively constant. Both factors increase the risk to depress organ blood flow and diuresis. Our objective was to test the hypothesis that high lung volume HFOV attenuates the postnatal reduction of extracellular volume in preterm infants by reducing plasma atrial natriuretic factor and diuresis.

DESIGN

Prospective, randomized, controlled clinical trial.

SETTING

University hospital, Level III neonatal intensive care unit.

PATIENTS

Premature infants <30 wks gestation requiring intubation for respiratory distress syndrome within the first 6 hrs of life; 15 infants (gestational age, 26 [24-29] wks, birth weight 814 [452-1340] g) were randomized to HFOV, 19 infants (gestational age 27 [24-39] wks, birth weight 930 [644-1490] g) to IPPV.

INTERVENTIONS

The randomized mode of ventilation was assigned within 1 hr after intubation. During HFOV mean airway pressure was increased as long as oxygenation improved and no lung overinflation was seen on chest radiograph. IPPV rates were > or =60/min.

MEASUREMENTS AND MAIN RESULTS

We measured extracellular volume (sucrose dilution) and atrial natriuretic factor on Day 1 and Day 3. Mean airway pressure, body weight, diuresis, and fluid intake were measured daily. During HFOV mean airway pressure was higher at 12 hrs (median 7 cm H2O vs. 4 cm H2O; p = .001) and 24 hrs (median 6 cm H2O vs. 3 cm H2O; p = .01). In both groups, extracellular volume decreased between Day 1 and Day 3 (HFOV from 428 +/- 126 mL to 344 +/- 145 mL [p = .003], IPPV from 466 +/- 108 mL to 414 +/- 124 mL [p = .01]) and diuresis increased (HFOV, from 2.5 +/- 1.7 to 4.6 +/- 0.9 mL/kg/hr [p = .001]; IPPV, from 2.8 +/- 1.6 to 4.2 +/- 1.0 mL/kg/hr [p = .01]). Plasma atrial natriuretic factor was not decreased in the HFOV group.

CONCLUSIONS

High lung volume HFOV as primary mode of ventilation in preterm infants <30 wks gestation did not result in unwanted fluid retention and a decrease in diuresis in the first days of life.

Randomized comparison of high-frequency ventilation with high-rate intermittent positive pressure ventilation in preterm infants with respiratory failure

OBJECTIVE

In a randomized, controlled, multicenter trial, we tested the hypothesis that high-frequency ventilation (HFV) with a high lung volume strategy results in fewer treatment failures than intermittent positive pressure ventilation (IPPV) with high rates and low peak inspiratory pressures.

STUDY DESIGN

Infants with a gestational age between >/=24 weeks and <30 weeks, requiring mechanical ventilation within 6 hours of birth, were randomly assigned to receive either IPPV or HFV until 240 hours after randomization, extubation, or meeting treatment failure criteria. Treatment failure, the primary end point, was determined when air leaks, an oxygenation index >35 to 45 (depending on gestational age), death, or chronic lung disease occurred. Chronic lung disease was defined as persistent requirement of mechanical ventilation, continuous positive airway pressure, or supplemental oxygen at a postmenstrual age of 36 weeks. Secondary end points included the incidence of intracranial hemorrhage.

RESULTS

The third scheduled interim analysis led to termination of the trial after recruitment of 284 infants. Treatment failure criteria were met by 46% of infants receiving IPPV and 54% of infants receiving HFV (1-tailed primary hypothesis, P =.92; 2-tailed chi2 test, P =.15). Air leaks occurred in 31% and 42% (P =.042), CLD in 23% and 25%, and grade 3-4 intracranial hemorrhage in 13% and 14% of IPPV-treated and HFV-treated patients, respectively. The mortality rate before discharge was 10% in both groups.

CONCLUSION

HFV with a high lung volume strategy did not cause less lung injury in preterm infants than IPPV with a high rate and low peak inspiratory pressures.

Pulmonary embolism and myocardial hypoxia during extracorporeal membrane oxygenation

The treatment of a newborn with severe meconium aspiration by venoarterial extracorporeal membrane oxygenation (ECMO) was complicated by myocardial hypoxia with a marked decrease of myocardial contractility. The onset of the cardiac hypoxia was related to a pulmonary artery embolus. The origin of the embolus was a deep femoral vein thrombosis, caused by a central vein catheter, which was inserted 1 day before ECMO by venous cutdown. The possible pathophysiology of myocardial hypoxia in this patient is discussed, especially with regard to myocardial perfusion, supporting the hypothesis of coronary perfusion occuring with blood from the left ventricle and not from the arterial cannula in the aorta.

25 years of respiratory support of newborn infants

Respiratory support of newborn infants has changed in the last 25 years, because of new knowledge of patho-physiology, controlled studies of respiratory therapy and the realisation of perinatal centers. Respiratory support has changed from the "blow in--suck out" approach, inevitably leading to severe atelectasis, high morbidity and mortality to a now very sophisticated therapy with reduced mortality and morbidity also in very-low-birth-weight infants, who were hopeless patients 25 years ago. Major milestones of this development were the introduction of continuous distending pressure to surfactant deficient lungs, the high-frequency positive pressure ventilation with fine tuning of inspiratory and expiratory times, adjusted to individual time constants and the substitution of artificial surfactant. Techniques for the future, like HFO, NO-inhalation, proportional assist ventilation and liquid ventilation are presently investigated.

Targeted inactivation of a tobacco intron-containing open reading frame reveals a novel chloroplast-encoded photosystem I-related gene

The chloroplast genome of all higher plants encodes, in its large single-copy region, a conserved open reading frame of unknown function (ycf3), which is split by two group II introns and undergoes RNA editing in monocotyledonous plants. To elucidate the function of ycf3 we have deleted the reading frame from the tobacco plastid genome by biolistic transformation. We show here that homoplasmic Deltaycf3 plants display a photosynthetically incompetent phenotype. Molecular analyses indicate that this phenotype is not due to a defect in any of the general functions of the plastid genetic apparatus. Instead, the mutant plants specifically lack detectable amounts of all photosystem I (PSI) subunits analyzed. In contrast, at least under low light conditions, photosystem II subunits are still present and assemble into a physiologically active complex. Faithful transcription of photosystem I genes as well as correct mRNA processing and efficient transcript loading with ribosomes in the Deltaycf3 plants suggest a posttranslational cause of the PSI-defective phenotype. We therefore propose that ycf3 encodes an essential protein for the assembly and/or stability of functional PSI units. This study provides a first example for the suitability of reverse genetics approaches to complete our picture of the coding capacity of higher plant chloroplast genomes.

Tissue-specific and differential editing of the two ycf3 editing sites in maize plastids

The hypothetical chloroplast open reading frame 3 (ycf3) of maize, consisting of three exons and two group II introns, contains two editing sites. Both of these sites were investigated with respect to the extent of editing in various tissues and different developmental stages. Northern blot analyses show nearly identical transcript patterns of ycf3 in all tissues investigated. In leaf plastids, both editing sites are completely edited, independent of light conditions and developmental stage. In non-leaf plastids, however, one editing site of ycf3 is only partially edited in unspliced transcripts and in one type of partially spliced transcripts. In different developmental stages of the same tissue, on the other hand, no differences in editing efficiency were found. These results indicate that, in partially spliced transcripts, different editing sites of one and the same gene can be edited with different efficiencies in a tissue-specific manner.

Occurrence of plastid RNA editing in all major lineages of land plants

RNA editing changes posttranscriptionally single nucleotides in chloroplast-encoded transcripts. Although much work has been done on mechanistic and functional aspects of plastid editing, little is known about evolutionary aspects of this RNA processing step. To gain a better understanding of the evolution of RNA editing in plastids, we have investigated the editing patterns in ndhB and rbcL transcripts from various species comprising all major groups of land plants. Our results indicate that RNA editing occurs in plastids of bryophytes, fern allies, true ferns, gymnosperms, and angiosperms. Both editing frequencies and editing patterns show a remarkable degree of interspecies variation. Furthermore, we have found that neither plastid editing frequencies nor the editing pattern of a specific transcript correlate with the phylogenetic tree of the plant kingdom. The poor evolutionary conservation of editing sites among closely related species as well as the occurrence of single species-specific editing sites suggest that the differences in the editing patterns and editing frequencies are probably due both to independent loss and to gain of editing sites. In addition, our results indicate that RNA editing is a relatively ancient process that probably predates the evolution of land plants. This supposition is in good agreement with the phylogenetic data obtained for plant mitochondrial RNA editing, thus providing additional evidence for common evolutionary roots of the two plant organellar editing systems.

Mutations of cytochrome b6 in Chlamydomonas reinhardtii disclose the functional significance for a proline to leucine conversion by petB editing in maize and tobacco

We have introduced a proline codon in place of a leucine codon at position 204 of the petB gene of Chlamydomonas reinhardtii. This gene modification mimics the presence of proline codons at the same position in the petB genes of maize and tobacco, which are subsequently edited to leucine codons at the RNA level. Following transformation, we observed no editing at this position in C. reinhardtii, independent of the type of proline codon we have used: the CCA codon, edited in maize, or a CCT codon. Strains carrying the introduced mutation were non phototrophic and displayed a block in photosynthetic electron transfer, consistent with a lack of cytochrome b6f activity. Thus the presence of a proline residue at position 204 in cytochrome b6 is detrimental to photosynthesis. We show that the mutant phenotype arose from a defective assembly of cytochrome b6f complexes and not from altered electron transfer properties in the assembled protein complex. Biochemical comparison of the proline-containing transformants with a cytochrome b6 mutant deficient in heme-attachment indicates that their primary defect is at the level of assembly of apocytochrome b6 with the bh heme, thereby preventing assembly of the whole cytochrome b6f complex.

Transcriptionally active chromosomes (TACs) of barley chloroplasts contain the alpha-subunit of plastome-encoded RNA polymerase

Transcriptionally active chromosomes (TACs) were isolated from mature chloroplasts of barley, from proplastids enriched in basal segments of barley primary foliage leaves, and from ribosome-deficient plastids of heat-bleached barley leaves. Immunological analysis with a specific antibody raised against the plastid rpoA gene product revealed that chloroplasts contain an immunoreactive protein of 38 kDa in the TAC fraction which appears to be identical to the alpha-subunit contained in the soluble RNA polymerase (sRNAP) fraction of the same chloroplasts. However, only traces of immunoreactive protein were detected in a TAC preparation derived from "proplastids". A positive correlation could be demonstrated between transcriptional activity and the amount of immunoreactive 38-kDa protein by analyzing different TAC fractions eluting at different times during gel filtration of a standard TAC preparation as well as in TAC preparations obtained under various detergent conditions.

In vivo dissection of cis-acting determinants for plastid RNA editing

Substitutional RNA editing changes single C nucleotides in higher plant chloroplast transcripts into U residues. To determine the cis-acting sequence elements involved in plastid RNA editing, we constructed a series of chloroplast transformation vectors harboring selected editing sites of the tobacco ndhB transcript in a chimeric context. The constructs were inserted into the tobacco plastid genome by biolistic transformation leading to the production of stable chimeric RNAs. Analysis of RNA editing revealed unexpected differences in the size of the essential cis elements or in their distance from the editing site. Flanking sequences of identical size direct virtually complete editing for one pair of editing sites, partial editing for a second and no editing at all for a third pair of sites. Serial 5' and 3' deletions allowed us to define the cis-acting elements more precisely and to identify a sequence element essential for editing site recognition. In addition, a single nucleotide substitution immediately upstream of an editing position was introduced. This mutation was found drastically and selectively to reduce the editing efficiency of the downstream editing site, demonstrating that position -1 is important for either site recognition or catalysis. Our results indicate that the editing of adjacent sites is likely to be mechanistically coupled. In no case did the presence in the plastome of the additional editing sites have any effect on the editing efficiency of the endogenous ndhB sites, indicating that the availability of site-specific trans-acting factors is not rate limiting.

Creation of a novel protein-coding region at the RNA level in black pine chloroplasts: the pattern of RNA editing in the gymnosperm chloroplast is different from that in angiosperms

The phenomenon of RNA editing has been found to occur in chloroplasts of several angiosperm plants. Comparative analysis of the entire nucleotide sequence of a gymnosperm [Pinus thunbergii (black pine)] chloroplast genome allowed us to predict several potential editing sites in its transcripts. Forty-nine such sites from 14 genes/ORFs were analyzed by sequencing both cDNAs from the transcripts and the corresponding chloroplast DNA regions, and 26 RNA editing sites were identified in the transcripts from 12 genes/ORFs, indicating that chloroplast RNA editing is not restricted to angiosperms but occurs in the gymnosperm, too. All the RNA editing events are C-to-U conversions; however, many new codon substitutions and creation of stop codons that have not so far been reported in angiosperm chloroplasts were observed. The most striking is that two editing events result in the creation of an initiation and a stop codon within a single transcript, leading to the formation of a new reading frame of 33 codons. The predicted product is highly homologous to that deduced from the ycf7 gene (ORF31), which is conserved in the chloroplast genomes of many other plant species.

Retinopathy of prematurity in infants of birth weight > 2000 g after haemorrhagic shock at birth

BACKGROUND

The risk of retinopathy of prematurity (ROP) is associated with low birth weight and low gestational age. For ROP screening examination is recommended in infants weighing < or = 1500 g or of less than 32 weeks' gestational age.

METHODS

From 1991 ROP screening was performed in 452 premature infants with either a birth weight < or = 1500 g (n = 303) or a birth weight > 1500 g (n = 149) and who required additional oxygen supplementation or underwent surgery with general anaesthesia before estimated term.

RESULTS

Unexpectedly, three infants with birth weights between 2080 and 2325 g and a gestational age of 32 or 33 weeks developed stage 2 or 3 ROP. One of these underwent cryocoagulation. In three infants, preterm birth was induced by sudden placental abruption with severe prenatal blood loss followed by haemorrhagic shock. The umbilical cord packed cell volume was reduced to 0.14-0.19 (normal 0.43-0.63). All three infants underwent surgery with general anaesthesia within the first weeks of life. Of the remaining 449 infants none with a birth weight > 1650 g developed any stage of ROP.

CONCLUSION

Severe prenatal blood loss requiring blood transfusions and surgery with general anaesthesia may induce higher stages of ROP even in infants with birth weights exceeding the usual screening criteria.

A promiscuous chloroplast DNA fragment is transcribed in plant mitochondria but the encoded RNA is not edited

The RNA editing processes in chloroplasts and mitochondira of higher plants show several similarities which are suggestive of common components and/or biochemical steps between the two plant organelles. The existence of various promiscuous DNA fragments of chloroplast origin in plant mitochondrial genomes allowed us to test the possibility that chloroplast sequences are also edited in mitochondria. An rpoB fragment transferred from chloroplasts to mitochondria in rice was chosen as it contains several editing sites, two of which match sequence motifs surrounding even non-homologous editing sites in both chloroplast and mitochondrial transcripts. Rice chloroplast and mitochondrial rpoB DNA and cDNA sequences were selectively amplified and the editing status of the cDNA sequences was determined. Three of the four potential rpoB editing sites previously detected in maize were found to be edited in the rice chloroplast rpoB transcript, whereas the fourth was found to remain unedited. In mitochondria, however, all four editing sites remain unmodified at the cDNA level. This indicates that the editing processes of higher plant mitochondria and chloroplasts are not identical and that organelle-specific factors are required for eliciting the respective editing events.

Occurrence of silent RNA editing in chloroplasts: its species specificity and the influence of environmental and developmental conditions

We have identified three new C-to-U RNA editing sites, one in atpF and two in atpA transcripts from tobacco chloroplasts. Two of them lead to amino acid substitutions to restore the conserved amino acid found in the corresponding genes of other plants. However, one editing site in the atpA transcript was found to take place partially at the third base of a serine codon (CUC_ to CUU_), thus not leading to an amino acid substitution. This is the first report of silent editing in chloroplasts. The extent of silent editing depends on plastid stage and light conditions, while editing as another site (found 4 nt upstream from the silent editing site) takes place constitutively even in non-photosynthetic cultured cells and bleached white seedlings grown in the presence of spectinomycin and streptomycin. In pea and spinach, despite a conservation in sequence, no editing at the site corresponding to the silent site in tobacco was found. This observation suggests that the silent editing detected in this study is species-specific.

[Extracorporeal membrane oxygenation of the newborn infant--an example of interdisciplinary project management]

In the last three years an interdisciplinary ECMO (extracorporeal membrane oxygenation) team has been set up at the Free University of Berlin in the Steglitz clinic. With this form of management, newborns only a few hours or days old who present with pulmonary failure can temporarily be managed with extracorporeal oxygenation until normal lung function is restored. The set up of this ECMO team, starting with experimental training in animal models, to the bedside clinical use is discussed. The following presentation of the first four cases managed here with ECMO aims to clarify the problems which may be encountered as well as amplifying its use as a life-saving measure.

Editing of the chloroplast ndhB encoded transcript shows divergence between closely related members of the grass family (Poaceae)

The ndhB-encoded transcript from barley chloroplasts deviates from the genomic ndhB sequence by nine C-to-U transitions, which is the maximum number of editing events for a chloroplast mRNA reported so far. Comparison with ndhB transcripts from other chloroplast species shows that six of the nine editing sites observed in barley are structurally and functionally conserved in maize, rice and tobacco. The remaining three sites, however, show divergent patterns of conservation even within the three members of the grass family. The conservation of two of these sites in tobacco but not in the closely related graminean species suggests that divergence of the ndhB editing sites is caused by the loss of preexisting editing sites rather than by gain of new sites.

The influence of lung injury due to mechanical ventilation on the initiation of ECMO

Before the entry criteria for extracorporeal membrane oxygenation (ECMO) are met, newborns may require aggressive mechanical ventilation which may result in lung injury. The question arises whether the presence of a pneumothorax in these infants plays a role in the prognosis. Of the 21 newborns transferred to our hospital for ECMO, 8 were treated with ECMO. 9 of the 21 newborns developed a pneumothorax with conventional ventilation and 6 of these 9 newborns subsequently required ECMO. Infants who developed a pneumothorax but did not meet ECMO criteria and remained in the oxygenation index (OI) range between 25 and 40 for more than 2 days had a poorer prognosis. If adequate oxygenation cannot be attained with acceptable mechanical ventilation and a more aggressive ventilation results in a pneumothorax, ECMO should be considered even if the oxygenation index is below 40.

Complete sequence of the maize chloroplast genome: gene content, hotspots of divergence and fine tuning of genetic information by transcript editing

The nucleotide sequence of the chloroplast (cp) DNA from maize (Zea mays) has been completed. The circular double-stranded DNA, which consists of 140,387 base-pairs, contains a pair of inverted repeat regions (IRA and IRB) with 22,748 base-pairs each, which are separated by a small and a large single copy region (SSC and LSC) of 12,536 and 82,355 base-pairs, respectively. The gene content and the relative positions of a total of 104 genes (70 peptide-encoding genes, 30 tRNA genes and four rRNA genes) are identical with the chloroplast DNA of the closely related species rice (Oryza sativa). A detailed analysis of the two graminean plastomes allows the identification of hotspots of divergence which predominate in one region containing a cluster of tRNA genes and in two regions containing degenerated reading frames. One of these length differences is thought to reflect a gene transfer event from the plastome to the nucleus, which is followed by progressive degradation of the respective chloroplast gene resulting in gene fragments. The other divergent plastome region seems to be due to the complete loss of a plastid gene and its functional substitution by a nuclear encoded eukaryotic homologue. The rate of neutral nucleotide substitutions is significantly reduced for protein coding genes located in the inverted repeat regions. This indicates that the existence of inverted repeat regions confers increased genetic stability of the genes positioned in these regions as compared to genes located in the two single copy regions. Editing events cause the primary structures of several transcripts to deviate from the corresponding genomic sequences by C to U transitions. The unambiguous deduction of amino acid sequences from the nucleotide sequences of the corresponding genes is, therefore, not possible. A survey of the 25 editing positions identified in 13 different transcripts of the maize plastome shows that representatives of all protein coding gene classes are subject to editing. A strong bias exists for the second codon position and for certain codon transitions. Based on the number and the codon transition types, and taking into account the frequency of putative editing sites in all peptide encoding genes and unidentified reading frames, a total number of only few more than the experimentally verified 25 editing sites encoded in the maize plastome is estimated. This corresponds to 0.13% of amino acid positions which cannot be derived from the corresponding codons present in the corresponding genes.

Structural and functional analysis of plastid genomes from parasitic plants: loss of an intron within the genus Cuscuta

The gene cluster rps12/rps7/psi ndhB of the plastome from the holoparasitic plant Cuscuta europaea has been analysed at the nucleotide level. A comparison with the homologous region of the plastome from the closely related parasite Cuscuta reflexa reveals a complete loss of the cis-spliced intron of the rps12 gene in addition to a drastic size reduction of the ndhB pseudogene. It is demonstrated by RT-PCR analysis that the entire gene cluster is transcribed in the form of a multicistronic transcript which also includes the sequences encoded by the ndhB pseudogene. A cDNA containing the correctly transpliced exon 1 of the rps12 transcript can also be amplified. This shows that trans-splicing of the rps12 transcript persists in the plastids of the holoparasite despite the loss of the cis-spliced intron and the loss of many other gene functions. The rps12 and rps7 genes, therefore, still appear to code for functional ribosomal proteins CS12 and CS7, respectively. The conservation of apparently intact ribosomal-protein genes from which correctly processed transcripts are produced is taken as evidence that the translational apparatus of the plastids is still functional and necessary for the expression of the genes remaining in the reduced plastome of a parasitic plant.

Introduction of a heterologous editing site into the tobacco plastid genome: the lack of RNA editing leads to a mutant phenotype

The psbF mRNA is edited in spinach plastids by a C to U conversion, changing a serine to a conserved phenylalanine codon. In tobacco at this position a phenylalanine codon is present at the DNA level, and the psbF mRNA here is not edited. To test if the psbF editing capacity is evolutionarily conserved, the tobacco psbF gene was modified to match the corresponding spinach sequence. The endogenous tobacco gene was replaced with the modified copy using biolistic transformation. We report here that the heterologous editing site remains unmodified in transplastomic tobacco plants. The lack of editing is associated with slower growth, lowered chlorophyll content and high chlorophyll fluorescence, a phenotype characteristic of photosynthetic mutants. This finding confirms that the editing of the psbF mRNA is an essential processing step for protein function and thus provides direct proof for the biological significance of plant organellar RNA editing. Given that a mutant phenotype is associated with the lack of editing, it seems likely that the evolutionary loss of the site-specific capacity for psbF editing was preceded by the mutation that eliminated the editing requirement.

Inefficient rpl2 splicing in barley mutants with ribosome-deficient plastids

Analysis of transcript accumulation and splicing in plastids of four nuclear mutants of barley revealed that the ribosomal protein L2 (rpl2) gene transcripts containing a group II intron remained entirely unspliced, whereas the intron of the ribosomal protein L16 (rpl16) gene (linked with the rpl2 gene in the same operon) was removed in the mutant plastids. Also, the transcripts of other genes containing group II introns (ribosomal protein S16 gene, rps16; NADH dehydrogenase ND2 gene, ndhB; cytochrome f gene, petD; and intron-containing reading frame 170, irf170) and of the tRNA for leucine, trnL (UAA), possessing the only chloroplast group I intron, were found to be spliced. The mutants used in this investigation are considered to be nonallelic; this excludes the possibility that a single nuclear gene is responsible for the impaired splicing of rpl2 transcripts. The mutants, however, have a severe deficiency in chloroplast ribosomes in common; this deficiency is evident from the lack of the essential ribosomal protein L2 and from an extremely low steady state level of plastid rRNAs. From these results, we conclude that a functioning translational apparatus of the organelle is a prerequisite for splicing of the chloroplast rpl2 class II intron but not for splicing of at least five other group II intron-containing transcripts. This provides genetic evidence for a chloroplast DNA-encoded component (e.g., a maturase) involved in the splicing of rpl2 pre-mRNA.

RNA editing of tobacco petB mRNAs occurs both in chloroplasts and non-photosynthetic proplastids

We found an RNA editing site in the protein coding region of tobacco (Nicotiana tabacum) petB transcripts. This editing (CCA to CUA) leads to an amino acid conversion from proline to leucine. It is observed not only in chloroplasts isolated from tobacco leaves but also in non-photosynthetic proplastids isolated from a tobacco cell culture. Also unspliced pre-mRNA shows complete editing. The editing site is the same as that recently observed in the maize petB transcripts which restores the codon for a highly conserved leucine residue, suggesting that RNA editing at this site is critical for the protein (cytochrome b6) function.

The role of RNA editing in conservation of start codons in chloroplast genomes

Open reading frames (ORFs), encoded by the plastid genomes of tobacco, liverwort, rice and maize were aligned with a view to studying the conservation of translational start and stop codons created by RNA editing of homologous genes. It became evident that most of the homologous ORFs have conserved translation start and stop signals at the gene level. However, some of the ORFs show differences with respect to extensions of their 3' and 5' terminal regions. For example, the proposed N-termini of the ndhD-encoded peptides from different plant species are very variable in length and amino-acid composition. Sequence analysis of ndhD and the corresponding cDNA shows that editing of an ACG triplet in tobacco, spinach and snapdragon leads to the creation of an AUG codon, corresponding to the start codon in other species. Conservation of translational start codons of plastome-encoded genes can, therefore, be achieved by editing of transcripts, and the definition of plastome-encoded ORFs must take potential editing events into consideration.

Complete RNA editing of unspliced and dicistronic transcripts of the intron-containing reading frame IRF170 from maize chloroplasts

The maize plastome harbors within the rps4-rps14 gene cluster the reading frame IRF170, which is interrupted by two introns. Although the function of the encoded peptide of 170 amino acids is not known, the conservation of IRF170 homologs in other plastomes is a strong indication that IRF170 is a functional gene. Amplification and sequence analyses of IRF170 specific cDNAs reveals two C-to-U editing events occurring within each of the first two exons. This situation allows an analysis of the temporal order between editing and splicing of a chloroplast transcript. By using intron-specific primer combinations, cDNAs derived from partially or even unspliced IRF170 transcripts could be amplified which in all cases showed complete editing. Complete editing was also observed with a cDNA derived from a transcript in which the proximal rps4 and the 5' half of IRF170-encoded sequences were still linked. This demonstrates that editing of the IRF170 transcript is an early processing step preceding both splicing and cleavage to monocistronic mRNA.

Editing of the chloroplast rpoB transcript is independent of chloroplast translation and shows different patterns in barley and maize

Sequence analysis of amplified cDNAs derived from the maize chloroplast rpoB transcript which encodes the beta subunit of a chloroplast specific, DNA dependent RNA polymerase reveals four C-to-U editing sites clustered within 150 nucleotides of the 5' terminal region of the rpoB message. These newly identified editing sites confirm the bias of chloroplast editing for certain codon transitions and for second codon positions which both appear suggestive for an involvement of the translational apparatus in the editing process. This supposition prompted us to investigate editing of the rpoB transcript from ribosome deficient, and hence protein synthesis deficient, plastids of the barley mutant albostrians. In this mutant editing is, however, not impaired at any of the editing sites functional in the barley wild type rpoB transcript. This demonstrates that chloroplast editing is neither linked to nor dependent on the chloroplast translational apparatus. As a further consequence any peptide components required for chloroplast editing must be encoded in the nuclear genome. In spite of strong sequence conservation only three of the four editing sites identified in the maize rpoB transcript are functional in barley. This indicates that sequences surrounding an editing site alone are not sufficient as determinants for the editing process in chloroplasts, but that trans-acting templates carrying the editing information for each individual site may also be required.

RNA editing in maize chloroplasts is a processing step independent of splicing and cleavage to monocistronic mRNAs

The psbB operon contained in the plastomes of higher plants consists of the genes psbB, psbH, petB and petD. The primary transcript of this operon is subject to a series of processing steps which include cleavages resulting in four monocistronic mRNAs and splicing of the petB and petD transcripts. A search for editing sites within the two latter transcripts from maize led us to the detection of one editing site within the petB coding region which is conserved at the DNA level in other graminean species and in tobacco. This shows that editing must be considered as an additional processing step of the psbB operon encoded primary transcript. As is evident from cDNA sequences derived from the dicistronic and/or unspliced petB/D transcripts which are completely edited, editing is an early step of mRNA processing which precedes both splicing and cleavage to the monocistronic mRNAs and which must, therefore, be independent of the latter two steps. This conclusion is confirmed by a similar observation with the editing site of the rpl2 transcript which is contained in the polycistronic transcript of the rpoA operon, although here only partial editing is observed for the unspliced dicistronic rpl23/rpl2 transcript.

Tissue- and stage-specific modulation of RNA editing of the psbF and psbL transcript from spinach plastids--a new regulatory mechanism?

The psbE operon of spinach chloroplasts, which includes the genes psbE, psbF, psbL and psbJ, encodes two RNA editing sites. One site corresponds to the initiation codon of the psbL transcript, as has been described earlier for the homologous transcript from tobacco, while at a second editing site, newly reported here, an internal phenylalanine codon of the psbF transcript is restored. Both these sites were investigated with respect to the extent of editing in spinach plastids at various developmental stages. The apparent existence of only completely edited transcripts in etioplasts and chloroplasts, indicates that light-induced processes are not acting as determinants in eliciting the editing process. Reduced editing is, however, observed in the psbF and psbL transcript from seeds and roots. This finding suggests that the RNA editing process is differentially down-regulated in leucoplasts and proplastids and that editing may, therefore, function as a regulatory device in plastid gene expression.

Identification of editing positions in the ndhB transcript from maize chloroplasts reveals sequence similarities between editing sites of chloroplasts and plant mitochondria

A comparison of the nucleotide sequences from genomic DNA and cDNA of the ndhB gene from maize chloroplasts shows that the ndhB transcript is edited by C-to-U transitions at six positions which appear to exist as editing sites also in the chloroplast ndhB genes from rice and tobacco but not from liverwort. In order to identify possible sequence determinants necessary for editing, the sequences surrounding the newly identified ndhB and previously identified ndhA editing sites were compared with each other and with editing sites observed in plant mitochondrial transcripts. Among the chloroplast editing sites two closely positioned ndhB sites show similarity by sharing a common octanucleotide. The existence of the identical octanucleotide in the ndhJ gene whose transcript is not edited at the respective position, shows, however, that this octanucleotide is not sufficient to elicit the editing process. On the other hand, several of the chloroplast editing sites show sequence similarities with certain sets of consensus sequences reported earlier for editing sites of plant mitochondria. This supports the view that the editing processes of both plant organelles share common components and/or mechanistic steps and that the consensus sequences are part of the determinants necessary for editing.

Internal editing of the maize chloroplast ndhA transcript restores codons for conserved amino acids

The NADH dehydrogenase subunit A (ndhA) gene from maize chloroplasts encodes a highly conserved peptide, which at several positions could be restored to consensus sequences by potential C-to-U editing of the codons involved. This gene was, therefore, chosen for analysis of its mRNA sequence in the form of amplified cDNA. A comparison of this cDNA sequence with the plastome-encoded ndhA sequence reveals four C-to-U editing sites, thereby demonstrating as a novel finding that chloroplast editing can also affect internal mRNA positions. All the edited codons restore amino acids that are conserved in the ndhA-encoded peptides of other chloroplast species. Alignment with homologous mitochondrial NADH-ubiquinone reductase subunit 1 (nad1) sequences of plant and even nonplant species shows that two of the editing positions restore universally conserved amino acids and that one editing site is even shared with nad1 mRNA of plant mitochondria. No editing sites could be detected in the cDNA derived from transcripts of the maize chloroplast RNA polymerase alpha-subunit (rpoA) gene.

RNA editing in tobacco chloroplasts leads to the formation of a translatable psbL mRNA by a C to U substitution within the initiation codon

The psbL gene which codes for a 38 amino acid peptide of photosystem II, together with the photosynthetic genes psbE and psbF, is contained in a conserved position of many species of higher plant plastomes. The alignment of the psbL nucleotide sequences from ten species shows strong conservation, which is indicative of a functional gene. The tobacco and spinach psbL genes have, however, an ACG codon instead of the initiator ATG codon observed in the homologous position of the other eight species. Evidence is presented that in tobacco chloroplasts a translatable psbL mRNA containing an AUG initiator codon is formed by a C to U editing of the ACG codon. This observation, following the previously reported editing of an rpl2 gene in maize chloroplasts, underlines a more widespread occurrence of this type of posttranscriptional mRNA modification and demonstrates its presence in a dicotyledon plant.

Editing of a chloroplast mRNA by creation of an initiation codon

Primary mRNA transcripts in several systems are edited by single base substitutions, small deletions or insertions to yield functional messenger RNA species. Mitochondrial mRNAs in particular, including those from plants, seem to be the subject of extensive editing, unlike mRNAs encoded by chloroplast DNA, for which the prediction of amino-acid sequence from the corresponding gene sequence is generally unambiguous. Occasionally, however, an ACG codon appears at the 5' terminus of chloroplast genes, where the initiation codon ATG would be expected. Here we present evidence for a C----U editing that is responsible for the conversion of the ACG codon to an AUG initiation codon in the mRNA transcript from the rpl2 gene of the maize plastome, showing that mRNA editing can also occur in chloroplasts.

Identification of in vivo processing intermediates and of splice junctions of tRNAs from maize chloroplasts by amplification with the polymerase chain reaction

Total RNA from chloroplasts of maize seedlings was used for polymerase chain reaction (PCR) mediated amplification of tRNA precursors and of mature tRNAs encoded by the two split tRNA genes of the ribosomal spacer (tRNA(lle)GAU and tRNA(Ala)UGC) and the single intron-containing tRNA(Gly)UCC gene. Sequence analysis of DNAs amplified from the mature tRNAs by combinations of exon specific primers allows unambiguous identification of the respective splice junctions. Primer combinations in which 5'- or 3'-flanking precursor tRNA sequences are included, leads to the amplification of processing intermediates in which 5'-terminal extensions are still present, whereas no PCR products corresponding to 3'-terminal extensions could be detected. From this it is concluded that in chloroplasts the 5'-terminal endonucleolytic cleavage by RNase P occurs as one of the final steps in the tRNA processing pathway of which the endonucleolytic cleavage at the 3' side probably occurs prior to the splicing of the intron sequences.

The ndhH genes of gramminean plastomes are linked with the junctions between small single copy and inverted repeat regions

The junctions JSA and JSB between the two inverted repeat regions IRA and IRB and the small single copy region of the maize chloroplast DNA have been identified by DNA sequencing. The JSA junction coincides with the initiation codon of the ndhH gene which is encoded by the adjacent region of the small single copy region. A comparison with the plastomes of rice, rye, tobacco and liverwort shows that linkage of this junction with the ndhH gene is specific for gramminean species. The amino acid sequences deduced from the ndhH genes show conserved histidine and cysteine residues which are likely to form a metal-binding domain.

Alternative base pairing between 5'- and 3'-terminal sequences of small subunit RNA may provide the basis of a conformational switch of the small ribosomal subunit

The compiled sequences of small subunit ribosomal RNAs have been screened for base complementary between 5'- and 3'-terminal regions. Highly conserved complementary sequences are found which allow formation of a helix between the two ends of 5 or 6 base pairs. This helix is composed of sequences from the loop region of the first 5'-terminal stem and from sequences immediately distal to the last stem (the Me2A-stem) of the 3' terminus and therefore allows a coaxial stacking with either of these two flanking stems. Formation of the 5'/3'-helical arrangement is, however, only possible at the cost of dissolving the 'pseudo-knot' helix between the 5'-terminal region and the internal region of small subunit RNA. It is postulated that the mutually exclusive conformational states are in dynamic equilibrium and that they correlate with distinct functional states of the small ribosomal subunit. The 'pseudo-knot' containing conformation with the 3'-terminal sequences more exposed is likely to represent the initiating state, whereas the 5'/3' terminal paired 'closed' conformation may represent the elongating state in which interaction with fortuitous ribosomal binding sequences of mRNAs is avoided.

Nucleotide sequence of the maize chloroplast rpo B/C1/C2 operon: comparison between the derived protein primary structures from various organisms with respect to functional domains

The genes (rpo B/C1/C2) coding for the beta, beta', beta" subunits of maize (Zea mays) chloroplast RNA polymerase have been located on the plastome and their nucleotide sequences established. The operon is part of a large inversion with respect to the tobacco and spinach chloroplast genomes and is flanked by the genes trnC and rps2. Notable features of the nucleotide sequence are the loss of an intron in rpoC1 and an insertion of approximately 450 bp in rpoC2 compared to the dicotyledons tobacco, spinach and liverwort. The derived amino acid sequence of this additional monocotyledon specific sequence is characterized by acidic heptameric repeat units containing stretches of glutamic acid, tyrosines and leucines with regular spacing. Other structural motifs, such as a nucleotide binding domain in the beta subunit and a zinc finger in the beta' subunit, are compared at the amino acid level throughout the RNA polymerase subunits with the enzymes from other organisms in order to identify functionally important conserved regions.

The gene for Klebsiella bacteriophage K11 RNA polymerase: sequence and comparison with the homologous genes of phages T7, T3, and SP6

We determined the nucleotide sequence of gene 1 of Klebsiella phage K11, which is a member of the T7 group of phages. The largest open reading frame corresponds to a polypeptide with 906 amino acids and a molecular weight of 100,383 daltons. The deduced amino acid sequence of this polypeptide shows 71% homology to the T7 RNA polymerase (the product of T7 gene 1), 72% homology to the T3 RNA polymerase and 27% homology to the SP6 RNA polymerase. Divergent evolution was clearly most pronounced in the amino-terminal portion.

Structure and expression of the gene coding for the alpha-subunit of DNA-dependent RNA polymerase from the chloroplast genome of Zea mays

The rpoA gene coding for the alpha-subunit of DNA-dependent RNA polymerase located on the DNA of Zea mays chloroplasts has been characterized with respect to its position on the chloroplast genome and its nucleotide sequence. The amino acid sequence derived for a 39 Kd polypeptide shows strong homology with sequences derived from the rpoA genes of other chloroplast species and with the amino acid sequence of the alpha-subunit from E. coli RNA polymerase. Transcripts of the rpoA gene were identified by Northern hybridization and characterized by S1 mapping using total RNA isolated from maize chloroplasts. Antibodies raised against a synthetic C-terminal heptapeptide show cross reactivity with a 39 Kd polypeptide contained in the stroma fraction of maize chloroplasts. It is concluded that the rpoA gene is a functional gene and that therefore, at least the alpha-subunit of plastidic RNA polymerase, is expressed in chloroplasts.

Functional in vivo verification in E. coli of promoter activities from the rDNA/tDNA(Val)(GAC) leader region of Zea mays chloroplasts

Restriction fragments containing upstream sequences of the rRNA operon from Zea mays chloroplasts were tested for promoter activity in vivo by insertion into an E. coli promoter-probe vector. The expression of this vector's reporter gene, which codes for alkaline phosphatase, was stimulated more than 1,500-fold upon linkage with the chloroplast rRNA promoter. Site specific mutagenesis of the invariant T of the -10 sequence of this promoter reduced the expression of the reporter gene to 2% of the wild type. This indicates that the chloroplast rRNA promoter, which directs transcriptional initiation 117 bp upstream of the 16S rRNA gene, is also active in the bacterial system. A restriction fragment further upstream containing the gene for tRNA(Val) (GAC) also showed strong promoter activity (29% as compared with the rRNA promoter). This promoter activity probably reflects the chloroplast promoter directing the synthesis of the tRNA(Val) (GAC) primary transcript. Surprisingly, this restriction fragment also displayed promoter activity (13% compared with the rRNA promoter) in reverse orientation.

The use of permanganate as a sequencing reagent for identification of 5-methylcytosine residues in DNA

The use of permanganate as a reagent for DNA sequencing by chemical degradation has been studied with respect to its specificity for 5-methylcytosine residues. At weakly acidic pH and room temperature, 0.2 mM potassium permanganate reacts preferentially with thymine, 5-methylcytosine, and to a lesser extent with purine residues, while cytosine remains essentially intact. Permanganate oxidation is, therefore, a suitable DNA sequencing reaction for positive discrimination between 5-methylcytosine and unmethylated cytosine.

The region distal to the rRNA operon from chloroplasts of maize contains genes coding for tRNA(Arg)(ACG) and tRNA (Asn)(GUU)

The nucleotide sequence distal to the rRNA operon from maize chloroplasts has been analyzed. It contains genes coding for tRNA(Arg)(ACG) and tRNA(Asn)(GUU). The tRNA(Arg)(ACG) gene, which is separated from the last rRNA gene of the rRNA operon, the 5S rDNA, by an intergenic region of 252 bp, has the same orientation as the rRNA operon. By S1 and primer extension mapping, the existence of transcripts from the entire 5S rDNA/tDNA(Arg)(ACG) intergenic region can be demonstrated. It is, therefore, concluded that tRNA(Arg)(ACG) represents a trailer tRNA which is cotranscribed with 5S rRNA as part of the primary rRNA transcript. The tDNA(Asn)(GUU), which is separated from tDNA(Arg)(ACG) by an intergenic region of 253 bp, has the opposite orientation with respect to the rRNA operon; it, therefore, represents a separate transcriptional unit whose promoter remains to be located. It is proposed that the two tRNA genes possess a common terminator region, which functions in both directions of transcription.