Expression pattern of Rhizobium etli ccmIEFH genes involved in c-type cytochrome maturation

In different bacterial species, ccmIEFH genes have been suggested to code for subunits of a bacterial haem-lyase catalyzing the covalent attachment of haem to c-type apoproteins. In Rhizobium etli CE3 there are two copies of ccmIEFH: one in the chromosome and the other located in plasmid pf. However, the null phenotype of chromosomal ccmF mutant indicates that the gene locus of plasmid pf is not functional. Two ccmI chromosomal mutants, previously isolated, produced detectable levels of c-type cytochromes under certain culture conditions in contrast with the ccmF mutant, suggesting that ccmF could be transcribed independently. The transcriptional organization of ccmIEFH operon was established. Two promoters from the chromosomal locus were mapped by primer extension, one located upstream of ccmI and the second located upstream of ccmF. The regulation of the expression of both promoters was studied using appropriate lacZ gene fusions (ccmI-lacZ and ccmEF-lacZ). The ccmI-lacZ gene fusion was expressed in complex medium, during exponential growth, under microaerobic conditions and in a R. etli mutant that accumulates reducing power, conditions where a higher respiration rate could be limited by c-type cytochrome content. The ccmEF-lacZ fusion was also primarily expressed in complex medium and under microaerophilic conditions. The finding of two independent promoters in this gene locus could suggest that the step catalyzed by CcmFH could be a rate-limiting step for c-type cytochrome assembly under certain culture conditions.

Gene amplification in Rhizobium: identification and in vivo cloning of discrete amplifiable DNA regions (amplicons) from Rhizobium leguminosarum biovar phaseoli

A genetic element that allows the positive selection of different genomic rearrangements was used to analyze DNA amplification in Rhizobium leguminosarum biovar phaseoli. Discrete amplifiable DNA regions (amplicons) were detected in different regions of the genome of the model strain CFN42, including the chromosome and several large plasmids. Amplicons were mobilized into Escherichia coli using a genetic approach that involves the introduction of an origin of replication active in E. coli and an origin of conjugal transfer into the amplifiable DNA regions of the Rhizobium genome. The strategy can be a valuable tool for studies on genome organization and function. We propose that amplicons define a structural characteristic of the genome that may play an important biological role.

Molecular cloning and characterization of the recA gene of Rhizobium phaseoli and construction of recA mutants

The Rhizobium phaseoli recA gene has been cloned by interspecific complementation of the Fec phenotype of bacteriophage lambda. The cloned gene restored the recombination proficiency and conferred resistance to DNA-damaging agents (methyl methanesulfonate and nitrofurantoin) to an Escherichia coli recA mutant. The direction of transcription and the localization of the recA gene were determined by mutagenesis with phage MudIIPR13 and heterologous hybridization with an E. coli recA probe. An R. phaseoli recA::Spcr mutation was introduced in two R. phaseoli strains by homogenotization. The R. phaseoli recA mutants were more sensitive to DNA-damaging agents and exhibited a 100-fold reduction in recombination frequency as compared with their parental strains. A deletion of the symbiotic plasmid abolishing nodulation was found at high frequency (10(-2)) in R. phaseoli CNF42. This event was recA dependent. In R. phaseoli CFN285, two events of symbiotic instability were found at high frequency (10(-3]: one was a deletion in the symbiotic plasmid, and the other was the loss of whole symbiotic plasmid. In the CFN285 recA::Spcr mutant, only the loss of the symbiotic plasmid was observed.

Structural complexity of the symbiotic plasmid of Rhizobium leguminosarum bv. phaseoli

The complete physical map of the symbiotic plasmid of Rhizobium leguminosarum bv. phaseoli strain CFN42 was established. The data support the concept that Rhizobium symbiotic genes are part of a complex genomic structure which contains a large amount of reiterated DNA sequences. This plasmid is a circular structure of 390 kb with approximately 10 families of internally reiterated DNA sequences of two to three elements each. One family includes two directly oriented nitrogenase operons situated 120 kb apart. We also found several stretches of pSym that are reiterated in other replicons of the cell. Localization of symbiotic gene sequences by heterologous hybridization revealed that nodABC sequences are separated in two regions, each of which contains a nod boxlike element, and it also suggested the presence of two copies of the nifA and nodD gene sequences. We propose that the complex structure of the symbiotic plasmid allows interactions between repeated DNA sequences which, in turn, might result in frequent rearrangements.

Amplification and deletion of a nod-nif region in the symbiotic plasmid of Rhizobium phaseoli

One remarkable characteristic of the genomes of some Rhizobium species is the frequent occurrence of rearrangements. In some instances these rearrangements alter the symbiotic properties of the strains. However, no detailed molecular mechanisms have been proposed for the generation of these rearrangements. To understand the mechanisms involved in the formation of rearrangements in the genome of Rhizobium phaseoli, we have designed a system which allows the positive selection for amplification and deletion events. We have applied this system to investigate the stability of the symbiotic plasmid of R. phaseoli. High-frequency amplification events were detected which increase the copy number of a 120-kb region carrying nodulation and nitrogen fixation genes two to eight times. Deletion events that affect the same region were also found, albeit at a lower frequency. Both kinds of rearrangements are generated by recombination between reiterated nitrogenase (nifHDK) operons flanking the 120-kb region.

[Influence of phosphatidylcholine/cholesterol molar ratios in liposomes on cholesterol reactivity with cholesterol:oxygen oxidoreductase]

The reactivity of sonicated phosphatidylcholine-cholesterol liposomes with cholesterol : oxygene oxydoreductase, an enzyme which catalyses the oxidation of the 3 beta hydroxyl group of cholesterol to a ketone group, is compared with that of ternary system phosphatidylcholine-cholesterol-Thesit. Regardless to the phosphatidylcholines nature and the phosphatidylcholine/cholesterol molar ratio (R), the enzymatic oxidation rate of liposomal cholesterol is slower than when the reaction is developed in the present of Thesit, a surfactif agent which destroyes the lamellar particles. This is true whether Thesit is added during preparation of dispersions or during incubation with cholesterol oxydase. The enzymatic oxydation rate of cholesterol of ternary systems phosphatidylcholine-cholesterol-Thesit is independent of the (R) value and the phosphatidylcholine fatty acid unsaturation, whereas that of phosphatidylcholine-cholesterol dispersions depends on these two parameters. The reaction rate increases in the order: dipalmitoylphosphatidylcholine to yolk egg phosphatidylcholines, and dioleylphosphatidylcholine. The optimal conditions for cholesterol oxidation were found to be R = 0.5. This result is not affected by the phosphatidylcholines nature. In order to explain these data, various hypotheses are considered. In particular, the weak liposomal cholesterol reactivity with cholesterol oxidase could result from an inhibitory effect on the enzyme-substrate combination due to the polar phosphorylcholine groups.

[A simple method for evaluation of the lecithin/sphingomyelin ratio in amniotic fluid. Variation in normal and abnormal pregnancies (138 cases) (author's transl)]

The determination of the lecithin/sphingomyelin ratio in amniotic fluid is a useful index for assessing fetal pulmonary maturity. The authors propose a simple and specific method : the amniotic lipids are extracted and thin layer chromatography is carried out in an acetic acid containing medium. The L/S ratio was studied and discussed in 138 normal and pathological cases.