Absence of insertions among spontaneous mutants of Salmonella typhimurium

A cis-encoded sRNA, Hfq and mRNA secondary structure act independently to suppress IS200 transposition

IS200 is found throughout Enterobacteriaceae and transposes at a notoriously low frequency. In addition to the transposase protein (TnpA), IS200 encodes an uncharacterized Hfq-binding sRNA that is encoded opposite to the tnpA 5'UTR. In the current work we asked if this sRNA represses tnpA expression. We show here that the IS200 sRNA (named art200 for antisense regulator of transposase IS200) basepairs with tnpA to inhibit translation initiation. Unexpectedly, art200-tnpA pairing is limited to 40 bp, despite 90 nt of perfect complementarity. Additionally, we show that Hfq and RNA secondary structure in the tnpA 5'UTR each repress tnpA expression in an art200-independent manner. Finally, we show that disrupting translational control of tnpA expression leads to increased IS200 transposition in E. coli. The current work provides new mechanistic insight into why IS200 transposition is so strongly suppressed. The possibility of art200 acting in trans to regulate a yet-unidentified target is discussed as well as potential applications of the IS200 system for designing novel riboregulators.

Global genome response of Escherichia coli O157∶H7 Sakai during dynamic changes in growth kinetics induced by an abrupt downshift in water activity

The present study was undertaken to investigate growth kinetics and time-dependent change in global expression of Escherichia coli O157∶H7 Sakai upon an abrupt downshift in water activity (aw). Based on viable count data, shifting E. coli from aw 0.993 to aw 0.985 or less caused an apparent loss, then recovery, of culturability. Exponential growth then resumed at a rate characteristic for the aw imposed. To understand the responses of this pathogen to abrupt osmotic stress, we employed an integrated genomic and proteomic approach to characterize its cellular response during exposure to a rapid downshift but still within the growth range from aw 0.993 to aw 0.967. Of particular interest, genes and proteins with cell envelope-related functions were induced during the initial loss and subsequent recovery of culturability. This implies that cells undergo remodeling of their envelope composition, enabling them to adapt to osmotic stress. Growth at low aw, however, involved up-regulating additional genes and proteins, which are involved in the biosynthesis of specific amino acids, and carbohydrate catabolism and energy generation. This suggests their important role in facilitating growth under such stress. Finally, we highlighted the ability of E. coli to activate multiple stress responses by transiently inducing the RpoE and RpoH regulons to control protein misfolding, while simultaneously activating the master stress regulator RpoS to mediate long-term adaptation to hyperosmolality. This investigation extends our understanding of the potential mechanisms used by pathogenic E. coli to adapt, survive and grow under osmotic stress, which could potentially be exploited to aid the selection and/or development of novel strategies to inactivate this pathogen.

Integrated transcriptomic and proteomic analysis of the physiological response of Escherichia coli O157:H7 Sakai to steady-state conditions of cold and water activity stress

An integrated transcriptomic and proteomic analysis was undertaken to determine the physiological response of Escherichia coli O157:H7 Sakai to steady-state conditions relevant to low temperature and water activity conditions experienced during meat carcass chilling in cold air. The response of E. coli during exponential growth at 25 °C a(w) 0.985, 14 °C a(w) 0.985, 25 °C a(w) 0.967, and 14 °C a(w) 0.967 was compared with that of a reference culture (35 °C a(w) 0.993). Gene and protein expression profiles of E. coli were more strongly affected by low water activity (a(w) 0.967) than by low temperature (14 °C). Predefined group enrichment analysis revealed that a universal response of E. coli to all test conditions included activation of the master stress response regulator RpoS and the Rcs phosphorelay system involved in the biosynthesis of the exopolysaccharide colanic acid, as well as down-regulation of elements involved in chemotaxis and motility. However, colanic acid-deficient mutants were shown to achieve comparable growth rates to their wild-type parents under all conditions, indicating that colanic acid is not required for growth. In contrast to the transcriptomic data, the proteomic data revealed that several processes involved in protein synthesis were down-regulated in overall expression at 14 °C a(w) 0.985, 25 °C a(w) 0.967, and 14 °C a(w) 0.967. This result suggests that during growth under these conditions, E. coli, although able to transcribe the required mRNA, may lack the cellular resources required for translation. Elucidating the global adaptive response of E. coli O157:H7 during exposure to chilling and water activity stress has provided a baseline of knowledge of the physiology of this pathogen.

Bile-induced DNA damage in Salmonella enterica

In the absence of DNA adenine methylase, growth of Salmonella enterica serovar Typhimurium is inhibited by bile. Mutations in any of the mutH, mutL, and mutS genes suppress bile sensitivity in a Dam(-) background, indicating that an active MutHLS system renders Dam(-) mutants bile sensitive. However, inactivation of the MutHLS system does not cause bile sensitivity. An analogy with Escherichia coli, in which the MutHLS system sensitizes Dam(-) mutants to DNA-injuring agents, suggested that bile might cause DNA damage. In support of this hypothesis, we show that bile induces the SOS response in S. enterica and increases the frequency of point mutations and chromosomal rearrangements. Mutations in mutH, mutL, or mutS cause partial relief of virulence attenuation in a Dam(-) background (50- to 100-fold by the oral route and 10-fold intraperitoneally), suggesting that an active MutHLS system reduces the ability of Salmonella Dam(-) mutants to cope with DNA-damaging agents (bile and others) encountered during the infection process. The DNA-damaging ability of bile under laboratory conditions raises the possibility that the phenomenon may be relevant in vivo, since high bile concentrations are found in the gallbladder, the niche for chronic Salmonella infections.

Phenotype microarray analysis of Escherichia coli K-12 mutants with deletions of all two-component systems

Two-component systems are the most common mechanism of transmembrane signal transduction in bacteria. A typical system consists of a histidine kinase and a partner response regulator. The histidine kinase senses an environmental signal, which it transmits to its partner response regulator via a series of autophosphorylation, phosphotransfer, and dephosphorylation reactions. Much work has been done on particular systems, including several systems with regulatory roles in cellular physiology, communication, development, and, in the case of bacterial pathogens, the expression of genes important for virulence. We used two methods to investigate two-component regulatory systems in Escherichia coli K-12. First, we systematically constructed mutants with deletions of all two-component systems by using a now-standard technique of gene disruption (K. A. Datsenko and B. L. Wanner, Proc. Natl. Acad. Sci. USA 97:6640-6645, 2000). We then analyzed these deletion mutants with a new technology called Phenotype MicroArrays, which permits assays of nearly 2,000 growth phenotypes simultaneously. In this study we tested 100 mutants, including mutants with individual deletions of all two-component systems and several related genes, including creBC-regulated genes (cbrA and cbrBC), phoBR-regulated genes (phoA, phoH, phnCDEFGHIJKLMNOP, psiE, and ugpBAECQ), csgD, luxS, and rpoS. The results of this battery of nearly 200,000 tests provided a wealth of new information concerning many of these systems. Of 37 different two-component mutants, 22 showed altered phenotypes. Many phenotypes were expected, and several new phenotypes were also revealed. The results are discussed in terms of the biological roles and other information concerning these systems, including DNA microarray data for a large number of the same mutants. Other mutational effects are also discussed.

Regulation of hisHF transcription of Aspergillus nidulans by adenine and amino acid limitation

The hisHF gene of Aspergillus nidulans encodes imidazole-glycerole-phosphate (IGP) synthase, consisting of a glutamine amidotransferase and a cyclase domain. The enzyme catalyzes the fifth and sixth steps of histidine biosynthesis, which results in an intermediate of the amino acid and an additional intermediate of purine biosynthesis. An A. nidulans hisHF cDNA complemented a Saccharomyces cerevisiae his7Delta strain and Escherichia coli hisH and hisF mutant strains. The genomic DNA encoding the hisHF gene was cloned and its sequence revealed two introns within the 1659-bp-long open reading frame. The transcription of the hisHF gene of A. nidulans is activated upon amino acid starvation, suggesting that hisHF is a target gene of cross pathway control. Adenine but not histidine, both end products of the biosynthetic pathways connected by the IGP synthase, represses hisHF transcription. In contrast to other organisms HISHF overproduction did not result in any developmental phenotype of the fungus in hyphal growth or the asexual life cycle. hisHF overexpression caused a significantly reduced osmotic tolerance and the inability to undergo the sexual life cycle leading to acleistothecial colonies.

Genetic linkage of the vanB2 gene cluster to Tn5382 in vancomycin-resistant enterococci and characterization of two novel insertion sequences

VanB-type vancomycin resistance is encoded by the vanB gene cluster, which disseminates by horizontal gene transfer and clonal spread of vancomycin-resistant enterococci (VRE). Genetic linkage of the vanB gene cluster to transposon Tn5382 and the insertion sequences IS16 and IS256-like has previously been shown. In this study linkage of defined vanB gene cluster subtypes to these elements was examined. All the vanB2 subtype strains studied (n=14) revealed co-hybridization of vanB and Tn5382, whereas the strains of vanB1 (n=8) and vanB3 (n=1) subtypes were Tn5382 negative. Conjugative cotransfer of the vanB2 gene cluster and Tn5382 was demonstrated for two strains. DNA sequencing of the vanX(B)-ORFC region in vanB2 strains confirmed that the vanB2 gene cluster is an integral part of Tn5382. No general pattern of linkage was observed with regard to IS16 and IS256-like. Two novel insertion sequences were identified in specific vanB2 subtype strains. (i) A 1611 bp element (ISEnfa110) was detected in the left flank of Tn5382. Its insertion site, lack of terminal inverted and direct repeats, and two conserved motifs in its putative transposase all conform to the conventions of the IS110 family. (ii) A 787 bp element (ISEnfa200) was detected in the vanS(B)-vanY(B) intergenic region. Its ORF encoded a putative protein with 60-70% identity to transposases of the IS200 family. No further copies of ISEnfa110 were found by colony hybridization of 181 enterococcal isolates, whereas ISEnfa200 was found in four additional vanB2 strains from the USA. The five strains had identical ISEnfa200 element insertion sites, and Tn5382 was located downstream from a pbp5 gene conferring high-level ampicillin resistance. These isolates showed related PFGE patterns, suggesting possible clonal spread of a VRE strain harbouring a Tn5382-vanB2-ISEnfa200 element linked to a pbp5 gene conferring ampicillin resistance.

Lack of hotspot targets: a constraint for IS30 transposition in Salmonella

IS30 is an insertion element common in E. coli strains but rare or absent in Salmonella. Transfer of the IS30-flanked transposon Tn2700 to Salmonella typhimurium was assayed using standard delivery procedures of bacterial genetics (conjugation and transduction). Tn2700 'hops' were rare and required transposase overproduction, suggesting the existence of host constraints for IS30 activity. Sequencing of three Tn2700 insertions in the genome of S. typhimurium revealed that the transposon had been inserted into sites with a low homology to the IS30 consensus target, suggesting that inefficient Tn2700 transposition to the Salmonella genome might be caused by a lack of hotspot targets. This view was confirmed by the introduction of an IS30 'hot target sequence', whose sole presence permitted Tn2700 transposition without transposase overproduction. Detection of IS30-induced DNA rearrangements in S. typhimurium provided further evidence that the element undergoes similar activities in E. coli and S. typhimurium. Thus, hotspot absence may be the main (if not the only) limitation for IS30 activity in the latter species. If these observations faithfully reproduce the scenario of natural populations, establishment of IS30 in the Salmonella genome may have been prevented by a lack of DNA sequences closely related to the unusually long (24 bp) IS30 consensus target.

Repression of IS200 transposase synthesis by RNA secondary structures

The IS 200 transposase, a 16 kDa polypeptide encoded by the single open reading frame (ORF) of the insertion element, has been identified using an expression system based on T7 RNA polymerase. In wild-type IS 200, two sets of internal inverted repeats that generate RNA secondary structures provide two independent mechanisms for repression of transposase synthesis. The inverted repeat located near the left end of IS 200 is a transcriptional terminator that terminates read-through transcripts before they reach the IS 200 ORF. The terminator is functional in both directions and may terminate >80% of transcripts. Another control operates at the translational level: transposase synthesis is inhibited by occlusion of the ribosome-binding site (RBS) of the IS 200 ORF. The RBS (5'-AGGGG-3') is occluded by formation of a mRNA stem-loop structure whose 3' end is located only 3 nt upstream of the start codon. This mechanism reduces transposase synthesis approximately 10-fold. Primer extension experiments with AMV reverse transcriptase have provided evidence that this stem-loop RNA structure is actually formed. Tight repression of transposase synthesis, achieved through synergistic mechanisms of negative control, may explain the unusually low transposition frequency of IS 200.

Cell division inhibition in Salmonella typhimurium histidine-constitutive strains: an ftsI-like defect in the presence of wild-type penicillin-binding protein 3 levels

Histidine-constitutive (Hisc) strains of Salmonella typhimurium undergo cell division inhibition in the presence of high concentrations of a metabolizable carbon source. Filaments formed by Hisc strains show constrictions and contain evenly spaced nucleoids, suggesting a defect in septum formation. Inhibitors of penicillin-binding protein 3 (PBP3) induce a filamentation pattern identical to that of Hisc strains. However, the Hisc septation defect is caused neither by reduced PBP3 synthesis nor by reduced PBP3 activity. Gross modifications of peptidoglycan composition are also ruled out. D-Cycloserine, an inhibitor of the soluble pathway producing peptidoglycan precursors, causes phenotypic suppression of filamentation, suggesting that the septation defect of Hisc strains may be caused by scarcity of PBP3 substrate.

Underground metabolism

All enzymes are able to use alternative substrates. When these are naturally occurring metabolites, an 'underground reaction' takes place. Examples are presented in which underground metabolism of this sort produces an observable phenotype. Although biological processes can be remarkably accurate, evolution has selected error rates far from perfect. It is suggested here that a certain level of metabolic inaccuracy, in addition to saving energy, may also confer an evolutionary advantage, for example by providing metabolic plasticity. Since underground reactions are unpredictable from DNA sequence data, caution is in order when interpreting correlations between genetic disorders and pathological syndromes.

Distinctive IS200 insertion between gyrA and rcsC genes in Salmonella typhi

While probing the vicinity of ompC, a copy of the IS200 insertion element was found between the gyrA and rcsC genes of Salmonella typhi, the causal agent of typhoid fever. This distinctive feature was conserved throughout 63 S. typhi isolates of different geographical origins and was absent from 46 other Salmonella serotypes, including those most associated with human infections, as well as from 19 other enteric bacteria. Furthermore, the location of this IS200 copy corresponds to a constant band, present throughout the 14 PstI S. typhi IS200 fingerprints, encompassing several Vi phage types. Interestingly, an apparently unrelated serotype not frequently associated with human disease, Salmonella weltevreden, contained an IS200 copy at the same position, although it was accompanied by an additional segment of cryptic DNA. On the basis of these findings, PCR assays were designed for molecular typing of S. typhi, and these are potentially useful in studying the epidemiology of typhoid fever.

Carbon starvation of Salmonella typhimurium does not cause a general increase of mutation rates

Mutation rates in bacteria can vary depending on the genetic target studied and the specific growth conditions of the cells. Here, two different methods were used to determine how rates of mutation to antibiotic resistance, auxotrophy, and prototrophy were influenced by carbon starvation on agar plates. The rate of mutation to rifampin resistance was increased by starvation as measured by fluctuation tests, similar to what has been reported previously for Escherichia coli. In contrast, the rates of mutation to various types of auxotrophy were unaffected or decreased as measured by both fluctuation tests and a repeated-streaking procedure. Similarly, the rates of reversion to prototrophy of his and lac nonsense and missense mutations were unaffected by starvation. Thus, mutation rates of different genetic targets can be affected differently by starvation and we conclude that carbon starvation is not generally mutagenic in Salmonella typhimurium.

Target choice determinants of the Tc1 transposon of Caenorhabditis elegans

The Tc1 transposon of Caenorhabditis elegans always integrates into the sequence TA, but some TA sites are preferred to others. We investigated a TA target site from the gpa-2 gene of C.elegans that was previously found to be preferred (hot) for Tc1 integration in vivo . This site with its immediate flanks was cloned into a plasmid, and remained hot in vitro , showing that sequences immediately adjacent to the TA dinucleotide determine this target choice. Further deletion mapping and mutagenesis showed that a 4 bp sequence on one side of the TA is sufficient to make a site hot; this sequence nicely fits the previously identified Tc1 consensus sequence for integration. In addition, we found a second type of hot site: this site is only preferred for integration when the target DNA is supercoiled, not when it is relaxed. Excision frequencies were relatively independent of the flanking sequences. The distribution of Tc1 insertions into a plasmid was similar when we used nuclear extracts or purified Tc1 transposase in vitro , showing that the Tc1 transposase is the protein responsible for the target choice.

The flagellin N-methylase gene fliB and an adjacent serovar-specific IS200 element in Salmonella typhimurium

The cloning and molecular genetic analysis of a locus mapping within the flagellar gene (fli) complex of Salmonella typhimurium is reported. A copy of the insertion element IS200 was located in a noncoding stretch of DNA upstream of the fliA gene. Comparative nucleotide sequence analysis showed that this copy of IS200 was 711 bp long and that its flanking regions contained no features common to other characterized insertion sites of this element. The element was located 37 bp downstream of an ORF whose product was shown by interspecific transfer and amino acid analysis to carry out N-methylation of selected lysine residues in Salmonella flagellin. The sequence and phenotype of this ORF identified it as fliB, encoding the only prokaryotic N-methylase acting on amino groups to have been characterized to date. It was found to be conserved among all clinically significant serovars of Salmonella. The IS200 insertion site is of particular interest since it was conserved in all but two rare evolutionary lines of S. typhimurium, and was absent from 85 Salmonella strains belonging to 37 other serovars. It is thus a phylogenetically significant marker at the serovar level.

Conserved structure of IS200 elements in Salmonella

Sequence analysis of three IS200 elements (two from Salmonella typhimurium, one from Salmonella abortusovis) reveals a highly conserved structure, with a length of 707-708 bp and absence of terminal repeats. IS200 contains an open-reading-frame (ORF) which potentially encodes a peptide of 151 amino acids, with a putative ribosome-binding-site properly placed upstream of the ORF. A potential RNA stem-loop structure that might occlude the ribosome-binding-site of the ORF is also found. Another conserved trait is a potential RNA hairpin which resembles a Rho-independent transcription terminator, located near one end of IS200. The junctions between IS200 and host DNA sequences are A+T-rich. Upon insertion, IS200 duplicates 1-2 bp of host DNA sequences. The observation that IS200 elements characterized as 'hops' are roughly identical to those residing in the Salmonella genome suggests that IS200 transposition is unlikely to generate inactive copies. If such is the case and many or all IS200 elements are active, the extremely low frequency of IS200 transposition may reflect the normal behavior of the element.

Strain typing with IS200 fingerprints in Salmonella abortusovis

A collection of Salmonella abortusovis isolates was examined for the presence of insertion element IS200. All proved to contain three or four copies of the element. One IS200 hybridization band of approximately 9 kb was found in all isolates, indicating that all S. abortusovis strains carry an IS200 element in similar or identical locations; this band can be potentially useful for serovar identification. S. abortusovis collection isolates from distinct geographic areas were highly polymorphic, suggesting that IS200 fingerprints might provide information on the geographic origin of S. abortusovis strains. Isolates obtained from the same geographic area (the island of Sardinia, Italy) were less polymorphic: all shared three constant IS200 hybridization bands, indicating that they derive from a single ancestor. Most strains analyzed contained an additional copy of IS200 in the variable region of the virulence plasmid. Certain Sardinian flocks proved to be infected by only one S. abortusovis strain, while others harbored two strains. Strain typing with IS200 fingerprints proved to be more reliable than plasmid analysis, because the latter yielded a high degree of polymorphism, even among isolates from the same flock.

Suppression of the pleiotropic effects of HisH and HisF overproduction identifies four novel loci on the Salmonella typhimurium chromosome: osmH, sfiW, sfiX, and sfiY

Insertion mutations that suppress some or all the pleiotropic effects of HisH and HisF overproduction were obtained by using transposons Tn10dTet and Tn10dCam. All suppressor mutations proved to be recessive, indicating that their effects were caused by loss of function; thus, the suppressors identify genes that are necessary to trigger the pleiotropic response when HisH and HisF are overproduced. Genetic mapping of the suppressor mutations identifies four novel loci on the Salmonella typhimurium genetic map. Mutations in osmH (min 49) behave as general suppressors that abolish all manifestations of the pleiotropic response. Mutations in sfiY (min 83) suppress cell division inhibition and thermosensitivity but not osmosensitivity. Mutations that suppress only cell division inhibition define another locus, sfiX (min 44). A fourth novel locus, sfiW (min 19), is also involved in cell division inhibition. The phenotype of sfiW mutations is in turn pleiotropic: they suppress cell division inhibition, make S. typhimurium unable to grow in minimal media, and cause slow growth and abnormal colony and cell shape. The inability of sfiW mutants to grow in minimal medium cannot be relieved by any known nutritional requirement or by the use of carbon sources other than glucose. The hierarchy of suppressor phenotypes and the existence of epistatic effects among suppressor mutations suggest a pathway-like model for the Hisc pleiotropic response.

Insertion sequence IS200 fingerprinting of Salmonella typhi: an assessment of epidemiological applicability

When Pst I-generated digests of genomic DNA from each of the type strains of 49 of the Vi phage types of Salmonella typhi were probed with a PCR-amplified IS200 gene probe, all strains were found to possess at least 11 IS200 elements carried on fragments in the range 24.2-1.2 kb. Fourteen fingerprints were identified but two patterns designated IS200Sty1 and IS200Sty2 predominated. In one strain, a plasmid-mediated IS200 element was identified. When IS200 fingerprinting was applied to epidemiologically-unrelated strains of S. typhi isolated in Ecuador, 3 patterns were identified in 10 strains belonging to 9 different phage types. It is concluded that Vi phage typing remains the method of choice for the primary differentiation of S. typhi but that IS200 fingerprinting may be of limited use in laboratories which do not have access to phage typing.

Location of IS200 on the genomic cleavage map of Salmonella typhimurium LT2

Locations of six Tn10s, closely linked to each of the six IS200s on the genomic cleavage map of Salmonella typhimurium LT2, were determined by digestion with XbaI and BlnI and separation of the fragments by pulsed-field gel electrophoresis; the locations were then further defined by P22-mediated joint transduction. The orientation of each IS200 with respect to its linked Tn10 was determined by Southern blotting. The locations of IS200-I, IS200-III, and IS200-V were confirmed to be close to sufD, melB, and purC, as previously indicated. IS200-II is jointly transduced with cysG. IS200-IV is near fliA; the linked Tn10 is inserted in fli, making the strain nonmotile. IS200-VI is jointly transduced with aspC but not with aroA. IS200 is transposed to a seventh site in some strains, while remaining in the other six locations described above. These data indicate that genome analysis by pulsed-field gel electrophoresis can locate the positions of Tn10s with accuracy sufficient to predict P22-mediated joint transduction.

The pleiotropic effects of his overexpression in Salmonella typhimurium do not involve AICAR-induced mutagenesis

Inhibition of cell division associated with overexpression of hisH and hisF in Salmonella typhimurium is strongly reminiscent of a cellular response to DNA damage. On these grounds, we investigated the involvement of a metabolite which appeared to represent a possible candidate for an endogenous mutagen: the base analog 5-amino-4-carboxamide imidazole riboside 5'-phosphate (AICAR), a by-product of HisH and HisF activity. However, we showed that AICAR is not an endogenous mutagen in S. typhimurium. Other types of DNA damage induced by his overexpression seem also unlikely, since similar mutation rates are found in hisO+ and hisOc strains. We also show that AICAR production is not involved in the pleiotropic effects of his overexpression, since these are still observed in strains devoid of AICAR. Thus inhibition of cell division resulting from HisH and HisF overexpression must operate through a mechanism unrelated to the role of these proteins in histidine biosynthesis.

Insertional mutations in mammals and mammalian cells

The retroposon sequences, their mechanisms of transposition and the occurrence of insertional mutation in the mammalian genome are reviewed. Insertional mutations fall into two broad categories: those due to the disruption of a gene following the physical integration of a foreign DNA sequence result in loss of gene product and would be expected to be associated with a recessive mutation. A second class of insertional mutation is well documented in which upon integration the promoter/enhancer activities inherent in the retroposon genome exert their influence on neighboring genes. This promoter/enhancer activity of integrated retroposons may have effects over relatively long distances and thus limit the possibilities of establishing an association between retroposon integration and mutation. It is emphasized that a systematic search for insertional mutations in the mammalian genome involves an extensive two-dimensional array of possible retroposon sequences and mutant alleles. Present results represent only a small portion of the total array. Future studies promise to be fruitful in efforts to isolate genes through insertional tagging, to characterize the mechanisms of retroposon transposition, as well as to study the stability of the mammalian genome.

Natural genetic engineering in evolution

The results of molecular genetics have frequently been difficult to explain by conventional evolutionary theory. New findings about the genetic conservation of protein structure and function across very broad taxonomic boundaries, the mosaic structure of genomes and genetic loci, and the molecular mechanisms of genetic change all point to a view of evolution as involving the rearrangement of basic genetic motifs. A more detailed examination of how living cells restructure their genomes reveals a wide variety of sophisticated biochemical systems responsive to elaborate regulatory networks. In some cases, we know that cells are able to accomplish extensive genome reorganization within one or a few cell generations. The emergence of bacterial antibiotic resistance is a contemporary example of evolutionary change; molecular analysis of this phenomenon has shown that it occurs by the addition rearrangement of resistance determinants and genetic mobility systems rather than by gradual modification of pre-existing cellular genomes. In addition, bacteria and other organisms have intricate repair systems to prevent genetic change by sporadic physicochemical damage or errors of the replication machinery. In their ensemble, these results show that living cells have (and use) the biochemical apparatus to evolve by a genetic engineering process. Future research will reveal how well the regulatory systems integrate genomic change into basic life processes during evolution.

Molecular analysis of an IS200 insertion in the gpt gene of Salmonella typhimurium LT2

A strain of Salmonella typimurium LT2 has been isolated which carries an insertion of approximately 700 bp in the gpt gene. The insertion in the gpt gene was shown to be the Salmonella-specific element IS200. The mutation in strain CR1 arose without selection during storage and is only the second phenotypically identified mutation caused by the insertion of IS200.

Transcriptional occlusion of transposon targets

In Salmonella typhimurium, insertion of transposons Tn5, Tn10 and bacteriophage Mu is inhibited by transcription of some target sequences. The transcription effects on Tn5 are large when the lac operon is a target but are limited to a slight effect on the hisG gene of the his operon. The Tn10 element shows target occlusion in both operons. Phage Mu has been shown previously to be inhibited for insertion into the lac operon. In the his operon Mu is only inhibited for insertion into the hisG gene. The variability of the inhibition effect from one sequence to another suggests site or regional specificity for transcription effects. Reducing the probability of insertion into transcribed sequences may be of selective importance to transposons since it reduces the risk of killing the host while maintaining the ability to transpose.