Davor Zahradka

The bacterium Deinococcus radiodurans is known to efficiently and accurately reassemble its genome after hundreds of DNA double-strand breaks (DSBs). Only at very large amounts of radiation-induced DSBs is this accuracy affected in the wild-type D. radiodurans, causing rearrangements in its genome structure. However, changes in its genome structure may also be possible during the propagation and storage of cell cultures. We investigate this possibility by listing structural differences between three completely sequenced genomes of D. radiodurans strains with a recent common ancestor—the type strain stored and sequenced in two different laboratories (of the ATCC 13939 lineage) and the first sequenced strain historically used as the reference (ATCC BAA-816). We detected a number of structural differences and found the most likely mechanisms behind them: (i) transposition/copy number change in mobile interspersed repeats—insertion sequences and small non-coding repeats, (ii) variable number of monomers within tandem repeats, (iii) deletions between long direct DNA repeats, and (iv) deletions between short (4–10 bp) direct DNA repeats. The most surprising finding was the deletions between short repeats because it indicates the utilization of a less accurate DSB repair mechanism in conditions in which a more accurate one should be both available and preferred. The detected structural differences, as well as SNPs and short indels, while being important footprints of deinococcal DNA metabolism and repair, are also a valuable resource for researchers using these D. radiodurans strains.

Homologous recombination repairs potentially lethal DNA lesions such as double-strand DNA breaks (DSBs) and single-strand DNA gaps (SSGs). In Escherichia coli, DSB repair is initiated by the RecBCD enzyme that resects double-strand DNA ends and loads RecA recombinase to the emerging single-strand (ss) DNA tails. SSG repair is mediated by the RecFOR protein complex that loads RecA onto the ssDNA segment of gaped duplex. In both repair pathways, RecA catalyses reactions of homologous DNA pairing and strand exchange, while RuvABC complex and RecG helicase process recombination intermediates. In this work, we have characterised cytological changes in various recombination mutants of E. coli after three different DNA-damaging treatments: (i) expression of I-SceI endonuclease, (ii) γ-irradiation, and (iii) UV-irradiation. All three treatments caused severe chromosome segregation defects and DNA-less cell formation in the ruvABC, recG, and ruvABC recG mutants. After I-SceI expression and γ-irradiation, this phenotype was efficiently suppressed by the recB mutation, indicating that cytological defects result mostly from incomplete DSB repair. In UV-irradiated cells, the recB mutation abolished cytological defects of recG mutants and also partially suppressed the cytological defects of ruvABC recG mutants. However, neither recB nor recO mutation alone could suppress the cytological defects of UV-irradiated ruvABC mutants. The suppression was achieved only by simultaneous inactivation of the recB and recO genes. Cell survival and microscopic analysis suggest that chromosome segregation defects in UV-irradiated ruvABC mutants largely result from defective processing of stalled replication forks. The results of this study show that chromosome morphology is a valuable marker in genetic analyses of recombinational repair in E. coli.

Background and purpose: Homologous recombination in Escherichia coli proceeds via two pathways, RecBCD and RecFOR, which use different enzymes for DNA end resection and loading of RecA recombinase. The postsynaptic reactions following RecA-mediated homologous pairing have mostly been studied within the RecBCD pathway. They involve RuvABC helicase/resolvase complex, RecG and RadA helicases that process recombination intermediates to produce recombinant DNA molecules. Also, RecG functionally interacts with the PriA protein in initiation of recombination-dependent replication. Here, we studied the individual and combined effects of ruvABC, recG and radA null mutations on transductional recombination in both pathways. The effect of the priA300 mutation, which acts as a suppressor of the recG mutation, was also tested. The goal was to characterize the postsynaptic stage of transductional recombination in more details, especially in the RecFOR pathway, which is less well-studied. Materials and methods: Phage P1vir-mediated transduction was used to measure recombination efficiency in a series of recombination mutants. The proA⁺ marker was used for selection in transductional crosses with various ΔproA recipients. Results: The ruvABC mutation moderately decreased recombination in both recombination pathways, while radA had no effect. The recG mutation reduced recombination in the RecBCD pathway but not in the RecFOR pathway. The strong recombination defect of recG radA double mutants in both pathways was completely suppressed by the priA300 mutation, and this suppression depended on the functional RuvABC complex. Conclusions: RecG and RadA proteins have a redundant role in transductional recombination via RecFOR pathway. In both recombination pathways, RecG and RadA functionally interact with PriA, probably during initiation of recombination-dependent replication.

Bacterial SSB proteins, as well as their eukaryotic RPA analogues, are essential and ubiquitous. They avidly bind single-stranded DNA and regulate/coordinate its metabolism, hence enabling essential DNA processes such as replication, transcription, and repair. The prototypic Escherichia coli SSB protein is encoded by an ssb gene. Although the ssb gene promoters harbor an SOS box, multiple studies over several decades failed to elucidate whether ssb gene expression is inducible and SOS dependent. The SOS regulon is comprised of about 50 genes, whose transcription is coordinately induced under stress conditions. Using quantitative real-time PCR, we determined the ssb gene expression kinetics in UV- and γ-irradiated E. coli and revealed that ssb gene expression is elevated in irradiated cells in an SOS-dependent manner. Additionally, the expression of the sulA gene was determined to indicate the extent of SOS induction. In a mutant with a constitutively induced SOS regulon, the ssb gene was overexpressed in the absence of DNA damage. Furthermore, we measured ssb gene expression by droplet digital PCR during unaffected bacterial growth and revealed that ssb gene expression was equal in wild-type and SOS⁻ bacteria, whereas sulA expression was higher in the former. This study thus reveals a complex pattern of ssb gene expression, which under stress conditions depends on the SOS regulon, whereas during normal bacterial growth it is unlinked to SOS induction. The E. coli ssb gene is SOS regulated in such a way that its basal expression is relatively high and can be increased only through stronger SOS induction. The remarkable SOS induction observed in undisturbed wild-type cells may challenge our notion of the physiological role of the SOS response in bacteria.

Genome stability in radioresistant bacterium Deinococcus radiodurans depends on RecA, the main bacterial recombinase. Without RecA, gross genome rearrangements occur during repair of DNA double-strand breaks. Long repeated (insertion) sequences have been identified as hot spots for ectopic recombination leading to genome rearrangements, and single-strand annealing (SSA) postulated to be the most likely mechanism involved in this process. Here, we have sequenced five isolates of D. radiodurans recA mutant carrying gross genome rearrangements to precisely characterize the rearrangements and to elucidate the underlying repair mechanism. The detected rearrangements consisted of large deletions in chromosome II in all the sequenced recA isolates. The mechanism behind these deletions clearly differs from the classical SSA; it utilized short (4–11 bp) repeats as opposed to insertion sequences or other long repeats. Moreover, it worked over larger linear DNA distances from those previously tested. Our data are most compatible with alternative end-joining, a recombination mechanism that operates in eukaryotes, but is also found in Escherichia coli. Additionally, despite the recA isolates being preselected for different rearrangement patterns, all identified deletions were found to overlap in a 35 kb genomic region. We weigh the evidence for mechanistic vs. adaptive reasons for this phenomenon.

Exponentially growingrecAmutant cells ofEscherichia colidisplay pronounced DNA degradation that starts at the sites of DNA damage and depends on RecBCD nuclease (ExoV) activity. As a consequence of this “reckless” DNA degradation, populations ofrecAmutants contain a large proportion of anucleate cells. We have found that both DNA degradation and anucleate-cell production are efficiently suppressed by mutations in thexonA(sbcB) andsbcDgenes. The suppressive effects of these mutations were observed in normally grown, as well as in UV-irradiated,recAcells. The products of thexonAandsbcDgenes are known to code for the ExoI and SbcCD nucleases, respectively. Since bothxonAandsbcDmutations are required for strong suppression of DNA degradation while individual mutations have only a weak suppressive effect, we infer that ExoI and SbcCD play partially redundant roles in regulating DNA degradation inrecAcells. We suggest that their roles might be in processing (blunting) DNA ends, thereby producing suitable substrates for RecBCD binding.

Escherichia coli cells with mutations in recBC genes are defective for the main RecBCD pathway of recombination and have severe reductions in conjugational and transductional recombination, as well as in recombinational repair of double-stranded DNA breaks. This phenotype can be corrected by suppressor mutations in sbcB and sbcC ( D ) genes, which activate an alternative RecF pathway of recombination. It was previously suggested that sbcB15 and Δ sbcB mutations, both of which inactivate exonuclease I, are equally efficient in suppressing the recBC phenotype. In the present work we reexamined the effects of sbcB15 and Δ sbcB mutations on DNA repair after UV and γ irradiation, on conjugational recombination, and on the viability of recBC ( sbcC ) cells. We found that the sbcB15 mutation is a stronger recBC suppressor than Δ sbcB , suggesting that some unspecified activity of the mutant SbcB15 protein may be favorable for recombination in the RecF pathway. We also showed that the xonA2 mutation, a member of another class of ExoI mutations, had the same effect on recombination as Δ sbcB , suggesting that it is an sbcB null mutation. In addition, we demonstrated that recombination in a recBC sbcB15 sbcC mutant is less affected by recF and recQ mutations than recombination in recBC Δ sbcB sbcC and recBC xonA2 sbcC strains is, indicating that SbcB15 alleviates the requirement for the RecFOR complex and RecQ helicase in recombination processes. Our results suggest that two types of sbcB -sensitive RecF pathways can be distinguished in E. coli , one that is activated by the sbcB15 mutation and one that is activated by sbcB null mutations. Possible roles of SbcB15 in recombination reactions in the RecF pathway are discussed.