What is the benefit to Escherichia coli of having multiple toxin-antitoxin systems in its genome?

The Escherichia coli K-12 chromosome encodes at least five proteic toxin-antitoxin (TA) systems. The mazEF and relBE systems have been extensively characterized and were proposed to be general stress response modules. On one hand, mazEF was proposed to act as a programmed cell death system that is triggered by a variety of stresses.

Biological roles of the Lon ATP-dependent protease.

The Lon ATP-dependent protease plays a major role in protein quality control. An increasing number of regulatory proteins, however, are being identified as Lon substrates, thus indicating that in addition to its housekeeping function, Lon plays an important role in regulating many biological processes in bacteria. This review presents and discusses the involvement of Lon in different aspects of bacterial physiology, including cell differentiation, sporulation, pathogenicity and survival under starvation conditions.

Control of bacteriophage mu lysogenic repression.

The transposable and temperate phage Mu infects Escherichia coli where it can enter the lytic life-cycle or reside as a repressed and integrated prophage. The repressor protein Rep is the key element in the lysis-lysogeny decision. We have analyzed the fate of Rep in different mutants by Western blotting under two conditions that can induce a lysogen: high temperature and stationary phase.

Overproduction of the Lon protease triggers inhibition of translation in Escherichia coli: involvement of the yefM-yoeB toxin-antitoxin system.

In Escherichia coli, the Lon ATP-dependent protease is responsible for degradation of several regulatory proteins and for the elimination of abnormal proteins. Previous studies have shown that the overproduction of Lon is lethal. Here, we showed that Lon overproduction specifically inhibits translation through at least two different pathways.

Starvation-induced Mucts62-mediated coding sequence fusion: a role for ClpXP, Lon, RpoS and Crp.

The formation of araB-lacZ coding sequence fusions in Escherichia coli is a particular type of chromosomal rearrangement induced by Mucts62, a thermoinducible mutant of mutator phage Mu. Fusion formation is controlled by the host physiology. It only occurs after aerobic carbon starvation and requires the phage-encoded transposase pA, suggesting that these growth conditions trigger induction of the Mucts62 prophage.

Multiple pathways for SOS-induced mutagenesis in Escherichia coli: an overexpression of dinB/dinP results in strongly enhancing mutagenesis in the absence of any exogenous treatment to damage DNA.

dinP is an Escherichia coli gene recently identified at 5.5 min of the genetic map, whose product shows a similarity in amino acid sequence to the E. coli UmuC protein involved in DNA damage-induced mutagenesis.

Different structures of selected and unselected araB-lacZ fusions.

Formation of araB-lacZ coding-sequence fusions is a key adaptive mutation system. Eighty-four independent araB-lacZ fusions were sequenced. All fusions carried rearranged MuR linker sequences between the araB and lacZ domains indicating that they arose from the standard intermediate of the well-characterized Mu DNA rearrangement process, the strand transfer complex (STC).

Adducts formed by the food mutagen 2-amino-3-methylimidazo(4,5-f) quinoline induce frameshift mutations at hot spots through an SOS-independent pathway.

The potency of 2-amino-3-methylimidazo(4,5-f)quinoline (IQ) adducts to induce -2, -1 and +1 frameshift mutations has been determined on specific target DNA sequences, namely short runs of alternating GpC sequences and short runs of guanines. The genetic control of the mutational processes has been analyzed using different Escherichia coli mutants, affected either in the control or in the mutagenesis pathway of the SOS system. We have shown that IQ adducts induce very efficiently both -1 and -2 frameshift mutations in E.

DNA damage and repair in mutagenesis and carcinogenesis: implications of structure-activity relationships for cross-species extrapolation.

Previous studies on structure-activity relationships (SARs) between types of DNA modifications and tumour incidence revealed linear positive relationships between the log TD50 estimates and s-values for a series of mostly monofunctional alkylating agents. The overall objective of this STEP project was to further elucidate the mechanistic principles underlying these correlations, because detailed knowledge on mechanisms underlying the formation of genotoxic damage is an absolute necessity for establishing guidance values for exposures to genotoxic agents. The analysis included: (1) the re-calculation and further extension of TD50 values in mmol/kg body weight for chemicals carcinogenic in rodents.

The roles of starvation and selective substrates in the emergence of araB-lacZ fusion clones.

The araB-lacZ fusion system has been a key case in the 'directed mutation' controversy. Fusions did not occur detectably during normal growth but formed readily after prolonged incubation on selective Ara-Lac medium. To distinguish the roles of starvation stress and selective substrates in coding sequence fusions, we applied sib selection and PCR technologies.

MucAB but not UmuDC proteins enhance -2 frameshift mutagenesis induced by N-2-acetylaminofluorene at alternating GC sequences.

N-2-acetylaminofluorene has been shown efficiently to induce both -1 and -2 frameshift mutations in Escherichia coli as well as in mammalian cells. In E. coli, the genetic characteristics of -1 and -2 frameshift mutations were found to be distinct.

Regulation of bacteriophage Mu transposition.

Bacteriophage Mu is a transposon and a temperate phage which has become a paradigm for the study of the molecular mechanism of transposition. As a prophage, Mu has also been used to study some aspects of the influence of the host cell growth phase on the regulation of transposition. Through the years several host proteins have been identified which play a key role in the replication of the Mu genome by successive rounds of replicative transposition as well as in the maintenance of the repressed prophage state.

A umuDC-independent SOS pathway for frameshift mutagenesis.

The chemical carcinogen N-acetoxy-N-2-acetylaminofluorene induces mainly frameshift mutations, which occur within two types of sequences (mutation hot spots): -1 frameshift mutations within contiguous guanine sequences and -2 frameshift mutations within alternating GC sequences such as the NarI and BssHII restriction site sequences. We have investigated the genetic control of mutagenesis at these sequences by means of a reversion assay using plasmids pW17 and pX2, which contain specific targets for contiguous guanine and alternating GC sequences, respectively. Our results suggest that mutations at these hot spot sequences are generated by two different genetic pathways, both involving induction of SOS functions.

Roles of recA mutant allele (recA495) in frameshift mutagenesis.

The chemical carcinogen N-acetoxy-N-2-acetylaminofluorene (N-AcO-AAF) induces frameshift mutations located within two types of specific sequences (mutation hot spots): i) contiguous guanine sequences and ii) alternating GC sequences. The genetic requirements of these frameshift events were investigated using specific reversion assays. AAF-induced -2 frameshift mutagenesis at alternating GC sequences is peculiar in that it requires a LexA- controlled function which is not UmuDC and occurs in the absence of RecA protein, provided the SOS regulon is derepressed.

Genetic control of the UV-induced SOS mutator effect in single- and double-stranded DNA phages.

The SOS hypothesis postulated that the mutator effect on undamaged DNA that generates phage-untargeted mutagenesis (UTM) results directly from the mechanism of targeted mutagenesis. RecA protein, which stimulates the cleavage of both the LexA repressor and UmuD protein, and the UmuDC gene products are required for UV-induced targeted mutagenesis. The use of phage lambda for analyzing UV-induced mutagenesis has permitted a distinction to be made between the mechanisms of targeted and untargeted mutagenesis, in that the two processes differ with respect to their genetic requirements for recA+ and umuDC+ genes.

Nature of the SOS mutator activity: genetic characterization of untargeted mutagenesis in Escherichia coli.

In Escherichia coli, induction of the SOS functions by UV irradiation or by mutation in the recA gene promotes an SOS mutator activity which generates mutations in undamaged DNA. Activation of RecA protein by the recA730 mutation increases the level of spontaneous mutation in the bacterial DNA. The number of recA730-induced mutations is greatly increased in mismatch repair deficient strains in which replication errors are not corrected.

GATC sequences, DNA nicks and the MutH function in Escherichia coli mismatch repair.

Circular heteroduplex DNAs of bacteriophage phi X174 have been constructed carrying either a G:T (Eam+/Eam3) or a G:A (Bam+/Bam16) mismatch and containing either two, one or no GATC sequences. Mismatches were efficiently repaired in wild-type Escherichia coli transfected with phi X174 heteroduplexes only when two unmethylated GATC sequences were present in phi X174 DNA. The requirements for GATC sequences in substrate DNA and for the E.

Specific strand loss in N-2-acetylaminofluorene-modified DNA.

N-2-Acetylaminofluorene (AAF), a well-known chemical carcinogen, when covalently linked to guanine residues constitutes a premutagenic lesion that is converted in vivo into frameshift mutations. In Escherichia coli, it is thought that -AAF adducts block the replication fork and that the mutagenic processing of the -AAF adducts is mediated by the SOS response. The construction in vitro of plasmids containing -AAF adducts in one strand only of a double-stranded DNA molecule enabled us to investigate the segregation of the strands and the mutagenicity of the lesions in vivo.

GATC sequence and mismatch repair in Escherichia coli.

The Escherichia coli mismatch repair system greatly improves DNA replication fidelity by repairing single mispaired and unpaired bases in newly synthesized DNA strands. Transient undermethylation of the GATC sequences makes the newly synthesized strands susceptible to mismatch repair enzymes. The role of unmethylated GATC sequences in mismatch repair was tested in transfection experiments with heteroduplex DNA of phage phi 174 without any GATC sequence or with two GATC sequences, containing in addition either a G:T mismatch (Eam+/Eam3) or a G:A mismatch (Bam+/Bam16).

Role of RecA protein in untargeted UV mutagenesis of bacteriophage lambda: evidence for the requirement for the dinB gene.

Untargeted UV mutagenesis of bacteriophage lambda--i.e., the increased recovery of lambda mutants when unirradiated lambda infects UV-irradiated Escherichia coli--is thought to be mediated by a transient decrease in DNA replication fidelity, generating mutations in the newly synthesized strands.

Processing of mispaired and unpaired bases in heteroduplex DNA in E. coli.

Bacteriophage lambda and phi X 174 DNAs, carrying sequenced mutations, have been used to construct in vitro defined species of heteroduplex DNA. Such heteroduplex DNAs were introduced by transfection, as single copies, into E. coli host cells.

Mechanism of SOS-induced targeted and untargeted mutagenesis in E. coli.

This paper retraces the evolution of hypotheses concerning mechanisms of SOS induced mutagenesis. Moreover, it reports some recent data which support a new model for the mechanism of targeted and untargeted mutagenesis in E. coli.

Involvement of DNA polymerase III in UV-induced mutagenesis of bacteriophage lambda.

It has been proposed that the mutation fixation processes stimulated by SOS induction result from an induced infidelity of DNA replication (Radman 1974). The aim of this study was to determine if mutator mutations in the E. coli DNA polymerase III might affect UV-induced mutagenesis.

Effect of umuC mutations on targeted and untargeted ultraviolet mutagenesis in bacteriophage lambda.

Mutagenesis of phage lambda towards clear-plaque phenotype (c+----c) results in two classes of mutants that can be distinguished genetically and morphologically. Indirect mutagenesis, i.e.