Functional complementation between chromosomal and plasmid mutagenic DNA repair genes in bacteria

The microbiology of mutability

Bacteria possessing elevated spontaneous mutation rates are prevalent in certain environments, which is a paradox because most mutations are deleterious. For example, cells with defects in the methyl-directed mismatch repair (MMR) system, termed mutators or hypermutators, are overrepresented in populations of bacterial pathogens, with the mutator trait hypothesized to be advantageous in the changing host enviroments faced during colonization and establishment of chronic infections. Error-prone DNA polymerases, such as polIV and polV, function in translesion DNA synthesis, a DNA damage response that ensures genome integrity with a cost of increased mutation. While the biochemical aspects of these mutability pathways are well understood, the biological impacts have received less attention. Here, an examination of bacterial mutability systems and specifically the ecological and evolutionary context resulting in the selection of these systems is carried out.

Identification of mucAB-like homologs on two IncT plasmids, R394 and Rts-1

Recent phylogenetic analysis of the superfamily of lesion-replicating DNA polymerases suggest that they can be broadly divided into four sub-groups comprised of UmuC-like, DinB-like, Rev1-like and Rad30-like proteins. The UmuC-like sub-family is best characterized at the genetic level and sequence analysis of eleven umu orthologs, residing on bacterial chromosomes or on self-transmissible R-plasmids allows further subdivision into five sub-groups (UmuDC, MucAB, ImpAB, RumAB and RulAB) based on amino acid sequence conservation. Some of these orthologs are apparently inactive in situ, but may promote increased mutagenesis and survival when subcloned and expressed from high-copy number plasmids. We were, therefore, interested in devising an assay that would identify umuC-like genes in situ in the absence of a functional assay. To this end, degenerate primers directed towards conserved amino acid regions within the UmuC-like sub-family of DNA polymerases were designed and used to identify mucAB-like operons on the IncT plasmids, R394 and Rts-1.Interestingly, DNA sequence analysis of an approximately 7kb region of R394 identified two LexA-regulated genes immediately downstream of mucAB((R394)) that are similar to the chromosomally-encoded Escherichia coli tus gene and the IncI plasmid-encoded impC gene, respectively. Analysis of the R394 and Rts-1 mucB genes revealed that both contain insertions which result in the expression of a truncated inactive MucB protein. While R394 was unable to restore mutagenesis functions to a DeltaumuDC E. coli strain, Rts-1 surprisingly promoted significant levels of MMS-induced SOS mutagenesis, raising the possibility that Rts-1 encodes another, yet unidentified, umu-like homolog.

The bacteriophage P1 HumD protein is a functional homolog of the prokaryotic UmuD'-like proteins and facilitates SOS mutagenesis in Escherichia coli

The Escherichia coli umuD and umuC genes comprise an operon and encode proteins that are involved in the mutagenic bypass of normally replication-inhibiting DNA lesions. UmuD is, however, unable to function in this process until it undergoes a RecA-mediated cleavage reaction to generate UmuD'. Many homologs of umuDC have now been identified. Most are located on bacterial chromosomes or on broad-host-range R plasmids. One such putative homolog, humD (homolog of umuD) is, however, found on the bacteriophage P1 genome. Interestingly, humD differs from other umuD homologs in that it encodes a protein similar in size to the posttranslationally generated UmuD' protein and not UmuD, nor is it in an operon with a cognate umuC partner. To determine if HumD is, in fact, a bona fide homolog of the prokaryotic UmuD'-like mutagenesis proteins, we have analyzed the ability of HumD to complement UmuD' functions in vivo as well as examined HumD's physical properties in vitro. When expressed from a high-copy-number plasmid, HumD restored cellular mutagenesis and increased UV survival to normally nonmutable recA430 lexA(Def) and UV-sensitive DeltaumuDC recA718 lexA(Def) strains, respectively. Complementing activity was reduced when HumD was expressed from a low-copy-number plasmid, but this observation is explained by immunoanalysis which indicates that HumD is normally poorly expressed in vivo. In vitro analysis revealed that like UmuD', HumD forms a stable dimer in solution and is able to interact with E. coli UmuC and RecA nucleoprotein filaments. We conclude, therefore, that bacteriophage P1 HumD is a functional homolog of the UmuD'-like proteins, and we speculate as to the reasons why P1 might require the activity of such a protein in vivo.

The virulence plasmid-encoded impCAB operon enhances survival and induced mutagenesis in Shigella flexneri after exposure to UV radiation

Upon exposure to UV radiation, Shigella flexneri SA100 displayed survival and mutation frequencies comparable to those of Escherichia coli AB1157, which contains a functional UmuDC error-prone DNA repair system. Survival of SA100 after UV irradiation was associated with the presence of the 220-kb virulence plasmid, pVP. This plasmid encodes homologues of ImpA and ImpB, which comprise an error-prone DNA repair system encoded on plasmid TP110 that was initially identified in Salmonella typhimurium, and ImpC, encoded upstream of ImpA and ImpB. Although the impB gene was present in representatives of all four species of Shigella, not all isolates tested contained the gene. Shigella isolates that lacked impB were more sensitive to UV radiation than isolates that contained impB. The nucleotide sequence of a 2.4-kb DNA fragment containing the imp operon from S. flexneri SA100 pVP was 96% identical to the imp operon from the plasmid TP110. An SA100 derivative with a mutation in the impB gene had reduced survival following UV irradiation and less UV-induced mutagenesis relative to the parental strain. We also found that S. flexneri contained a chromosomally encoded umuDC operon; however, the umuDC promoter was not induced by exposure to UV radiation. This suggests that the imp operon but not the umuDC operon contributes to survival and induced mutagenesis in S. flexneri following exposure to UV radiation.

Inhibition of homologous recombination by the plasmid MucA'B complex

By its functional interaction with a RecA polymer, the mutagenic UmuD'C complex possesses an antirecombination activity. We show here that MucA'B, a functional homolog of the UmuD'C complex, inhibits homologous recombination as well. In F- recipients expressing MucA'B from a Ptac promoter, Hfr x F- recombination decreased with increasing MucA'B concentrations down to 50-fold. In damage-induced pKM101-containing cells expressing MucA'B from the native promoter, recombination between a UV-damaged F lac plasmid and homologous chromosomal DNA decreased 10-fold. Overexpression of MucA'B together with UmuD'C resulted in a synergistic inhibition of recombination. RecA[UmuR] proteins, which are resistant to UmuD'C inhibition of recombination, are inhibited by MucA'B while promoting MucA'B-promoted mutagenesis efficiently. The data suggest that MucA'B and UmuD'C contact a RecA polymer at distinct sites. The MucA'B complex was more active than UmuD'C in promoting UV mutagenesis, yet it did not inhibit recombination more than UmuD'C does. The enhanced mutagenic potential of MucA'B may result from its inherent superior capacity to assist DNA polymerase in trans-lesion synthesis. In the course of this work, we found that the natural plasmid pKM101 expresses around 45,000 MucA and 13,000 MucB molecules per lexA(Def) cell devoid of LexA. These molecular Muc concentrations are far above those of the chromosomally encoded Umu counterparts. Plasmid pKM101 belongs to a family of broad-host-range conjugative plasmids. The elevated levels of the Muc proteins might be required for successful installation of pKM101-like plasmids into a variety of host cells.

Linkage map of Escherichia coli K-12, edition 10: the traditional map

This map is an update of the edition 9 map by Berlyn et al. (M. K. B. Berlyn, K. B. Low, and K. E. Rudd, p. 1715-1902, in F. C. Neidhardt et al., ed., Escherichia coli and Salmonella: cellular and molecular biology, 2nd ed., vol. 2, 1996). It uses coordinates established by the completed sequence, expressed as 100 minutes for the entire circular map, and adds new genes discovered and established since 1996 and eliminates those shown to correspond to other known genes. The latter are included as synonyms. An alphabetical list of genes showing map location, synonyms, the protein or RNA product of the gene, phenotypes of mutants, and reference citations is provided. In addition to genes known to correspond to gene sequences, other genes, often older, that are described by phenotype and older mapping techniques and that have not been correlated with sequences are included.

Arrested DNA replication in Xenopus and release by Escherichia coli mutagenesis proteins

Xenopus oocytes and oocyte nuclear extracts repair ultraviolet photoproducts on double-stranded (ds) DNA and replicate single-stranded (ss) to ds DNA. M13 ss DNA molecules containing cyclobutane pyrimidine dimers were maintained but not replicated in Xenopus oocytes yet were replicated in progesterone-matured oocytes. The replication arrest functioned only in cis. The replication arrest was alleviated by injection into oocytes of messenger RNAs encoding the prokaryotic mutagenesis proteins UmuD'C or MucA'B. These results may help explain how cells stabilize repair or replication events on DNA with unrepairable lesions.

Analysis of chimeric UmuC proteins: identification of regions in Salmonella typhimurium UmuC important for mutagenic activity

Unlike Escherichia coli, the closely related bacterium Salmonella typhimurium is relatively unresponsive to the mutagenic effects of DNA-damaging agents. Previous experiments have suggested that these phenotypic differences might result from reduced activity of the S. typhimurium UmuC protein. To investigate this possibility, we have taken advantage of the high degree of homology between the UmuC proteins of E. coli and S. typhimurium and have constructed a series of plasmid-encoded chimeric proteins. The possibility that the phenotypic differences might be due to differential expression of the respective UmuC proteins was eliminated by constructing chimeric proteins that retained the first 25 N-terminal amino acids of either of the UmuC proteins (and presumably the same translational signals), but substituting the remaining 397 C-terminal amino acids with the corresponding segments from the reciprocal operon. Constructs expressing mostly E. coli UmuC were moderately proficient for mutagenesis whereas those expressing mostly S. typhimurium UmuC exhibited much lower frequencies of mutation, indicating that the activity of the UmuC protein of S. typhimurium is indeed curtailed. The regions responsible for this phenotype were more precisely localized by introducing smaller segments of the S. typhimurium UmuC protein into the UmuC protein of E. coli. While some regions could be interchanged with few or no phenotypic effects, substitution of residues 212-395 and 396-422 of E. coli UmuC with those from S. typhimurium resulted in reduced mutability, while substitution of residues 26-59 caused a dramatic loss of activity. We suggest, therefore, that the primary cause for the poor mutability of S. typhimurium can be attributed to mutations located within residues 26-59 of the S. typhimurium UmuC protein.

Construction and characterization of two lexA mutants of Salmonella typhimurium with different UV sensitivities and UV mutabilities

Salmonella typhimurium has a SOS regulon which resembles that of Escherichia coli. recA mutants of S. typhimurium have already been isolated, but no mutations in lexA have been described yet. In this work, two different lexA mutants of S. typhimurium LT2 have been constructed on a sulA background to prevent cell death and further characterized. The lexA552 and lexA11 alleles contain an insertion of the kanamycin resistance fragment into the carboxy- and amino-terminal regions of the lexA gene, respectively. SOS induction assays indicated that both lexA mutants exhibited a LexA(Def) phenotype, although SOS genes were apparently more derepressed in the lexA11 mutant than in the lexA552 mutant. Like lexA(Def) of E. coli, both lexA mutations only moderately increased the UV survival of S. typhimurium, and the lexA552 strain was as mutable as the lexA+ strain by UV in the presence of plasmids encoding MucAB or E. coli UmuDC (UmuDCEc). In contrast, a lexA11 strain carrying any of these plasmids was nonmutable by UV. This unexpected behavior was abolished when the lexA11 mutation was complemented in trans by the lexA gene of S. typhimurium. The results of UV mutagenesis correlated well with those of survival to UV irradiation, indicating that MucAB and UmuDCEc proteins participate in the error-prone repair of UV damage in lexA552 but not in lexA11. These intriguing differences between the mutagenic responses of lexA552 and lexA11 mutants to UV irradiation are discussed, taking into account the different degrees to which the SOS response is derepressed in these mutants.

The spectra of base substitutions induced by the impCAB, mucAB and umuDC error-prone DNA repair operons differ following exposure to methyl methanesulfonate

We have used the lacZ reversion assay to study the mutation spectra induced by the Escherichia coli chromosomal umuDC operon and of its two plasmid-borne analogues impCAB and mucAB following exposure of cells to UV light and methyl methanesulfonate (MMS). We have shown that the impCAB, mucAB and umuDC operons all produce a similar response to UV light which results almost exclusively in AT-->GC transitions. However, we found that the three operons produced different responses to alkylating agents. We found that with MMS the chromosomal umuDC operon produced almost exclusively AT-->GC transitions, whilst both mucAB and impCAB produced predominantly transversions. In the case of the impCAB operon the mutation spectrum contained more AT-->TA than GC-->TA transversions; this balance was reversed with mucAB. The effect of the copy number of the error-prone DNA repair operons upon the mutagenic spectra was also studied. The results obtained suggest that the copy number of the imp operon does not greatly affect the specificity of base substitutions observed. However, an increase in the copy number of the umuDC operon greatly affected the specificity of base substitution, such that virtually no transitions were produced and the spectrum was dominated by GC/AT-->TA transversions. It appears that the three error-prone DNA repair operons impCAB, mucAB and umuDC, despite showing strong structural and functional homologies, can display major differences in the spectrum of base changes induced during mutagenesis.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of the umu-complementing operon from R391

In addition to conferring resistances to antibiotics and heavy metals, certain R factors carry genes involved in mutagenic DNA repair. These plasmid-encoded genes are structurally and functionally related to the chromosomally encoded umuDC genes of Escherichia coli and Salmonella typhimurium. Three such plasmid operons, mucAB, impCAB, and samAB, have been characterized at the molecular level. Recently, we have identified three additional umu-complementing operons from IncJ plasmid R391 and IncL/M plasmids R446b and R471a. We report here the molecular characterization of the R391 umu-complementing operon. The nucleotide sequence of the minimal R plasmid umu-complementing (rum) region revealed an operon of two genes, rumA(R391) and rumB(R391), with an upstream regulatory signal strongly resembling LexA-binding sites. Phylogenetic analysis revealed that the RumAB(R391) proteins are approximately equally diverged in sequence from the chromosomal UmuDC proteins and the other plasmid-encoded Umu-like proteins and represent a new subfamily. Genetic characterization of the rumAB(R391) operon revealed that in recA+ and recA1730 backgrounds, the rumAB(R391) operon was phenotypically indistinguishable from mucAB. In contrast, however, the rumAB(R391) operon gave levels of mutagenesis that were intermediate between those given by mucAB and umuDC in a recA430 strain. The latter phenotype was shown to correlate with the reduced posttranslational processing of the RumA(R391) protein to its mutagenically active form, RumA'(R391). Thus, the rumAB(R391) operon appears to possess characteristics that are reminiscent of both chromosome and plasmid-encoded umu-like operons.

Involvement of umuDCST genes in nitropyrene-induced -CG frameshift mutagenesis at the repetitive CG sequence in the hisD3052 allele of Salmonella typhimurium

Expression of the umuDC operon is required for UV and most chemical mutagenesis in Escherichia coli. The closely related species Salmonella typhimurium has two sets of umuDC-like operons, umuDCST on the chromosome and samAB on a 60-MDa cryptic plasmid. The roles of the umuDC-like operons in chemically induced frameshift mutagenesis of the hisD3052 allele of S. typhimurium were investigated. Introduction of a pBR322-derived plasmid carrying umuDCST increased the rate of reversion of hisD3052, following treatment with 1-nitropyrene (1-NP) or 1,8-dinitropyrene (1,8-DNP) tenfold and fivefold, respectively, whereas it did not substantially increase the rate of reversion induced by other frameshift mutagens, i.e. 2-nitrofluorene (2-NF) and 2-amino-3-methyldipyrido[1,2-a:3',2'-d]imidazole (Glu-P-1). Introduction of a pBR322-derived plasmid carrying samAB did not increase the incidence of reversion of hisD3052 observed with any of the mutagens examined. Deletion of umuDCST substantially lowered the reversion rate induced by 1-NP or 1,8-DNP, but it did not affect reversion induced by 2-NF, Glu-P-1 or N-hydroxyacetylaminofluorene (N-OH-AAF). Deletion of samAB had little impact on reversion incidence induced by any of the five frameshift mutagens. DNA amplification using the polymerase chain reaction technique followed by restriction enzyme analysis using BssHII, suggested that the mutations induced by the five frameshift mutagens were all CG deletions at the CGCGCGCG sequence in hisD3052. These results suggest that umuDCST, but not samAB, is involved in the -2 frameshift mutagenesis induced by 1-NP and 1,8-DNP at the repetitive CG sequence, whereas neither operon participates in induction of the same type of mutations by 2-NF, Glu-P-1 or N-OH-AAF.

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. The -1 frameshift mutation pathway occurs at monotonous runs of G residues (i.e. GGG-->GG). This pathway exhibits the same genetic requirements as UV light-induced base substitution mutagenesis. Indeed, optimal mutagenesis requires the expression of both UmuDC and the activated form of RecA. The -2 frameshift mutation pathway operates at short alternating GpC sequences, such as the NarI sequence (i.e. GGCGCC-->GGCC). In contrast to the -1 frameshift mutation pathway, optimal induction does not require the UmuDC and RecA proteins. This pathway involves a LexA-repressed function tentatively called Npf (for NarI processing factor). In this paper, we show that MucAB efficiently stimulates the -2 frameshift mutation pathway. However, unlike the Npf pathway, MucAB-mediated stimulation requires expression of the RecA protein.

The groE gene products of Escherichia coli are dispensable for mucA+B(+)-dependent UV mutagenesis

UV mutagenesis in Escherichia coli requires the groES+EL+ chaperonins as well as the umuD+C+ SOS-regulated genes. GroES and GroEL appear to be required to stabilize UmuC. The mucA+B+ genes, which are encoded on a broad host range plasmid, are functionally analogous and structurally similar to the umuD+C+ genes of E. coli. While these gene pairs are quite similar, differences have been reported in the functioning of these gene products. We tested whether mucA+B+ function requires the groE+ gene products as well. We show that mucA+B(+)-induced UV mutagenesis, UV resistance, phage reactivation and cold sensitivity do not require the groE+ heat shock genes. These findings suggest that the requirement of UmuC for groES+EL+ function is not shared by its analog, MucB.

Isolation and prevalidation of an Escherichia coli tester strain for the use in mechanistic and metabolic studies of genotoxins

We have isolated an Escherichia coli tester strain for the use in mechanistic and metabolic studies of genotoxins. We started with one of the more used and better characterized E. coli K-12 laboratory strains, AB1157. We isolated a lipopolysaccharide defective mutant of strain AB1886 which is an excision repair deficient derivative of AB1157 and introduced a newly constructed plasmid pKR11, encoding mucAB, resulting in strain MR2101/pKR11. A genotoxicity assay was designed, monitoring the reversion to arginine prototrophy and a preliminary validation was carried out against Ames tester strain TA100 with a set of diagnostic compounds. The results seem to indicate that strain MR2101/pKR11 is an adequate tester strain which can be a useful tool in mechanistic studies. Moreover, this strain can serve as mother strain to isolate improved and more specialized tester strains.

SOS mutagenesis

In Escherichia coli, UV and many chemicals appear to cause mutagenesis by a process of translesion synthesis that requires some form of DNA polymerase III and the SOS-regulated proteins UmuD, UmuC and RecA. An analysis of SOS mutagenesis offers insights into the molecular basis of induced mutagenesis and into mechanisms of DNA damage tolerance.

Replication of damaged DNA and the molecular mechanism of ultraviolet light mutagenesis

On UV irradiation of Escherichia coli cells, DNA replication is transiently arrested to allow removal of DNA damage by DNA repair mechanisms. This is followed by a resumption of DNA replication, a major recovery function whose mechanism is poorly understood. During the post-UV irradiation period the SOS stress response is induced, giving rise to a multiplicity of phenomena, including UV mutagenesis. The prevailing model is that UV mutagenesis occurs by the filling in of single-stranded DNA gaps present opposite UV lesions in the irradiated chromosome. These gaps can be formed by the activity of DNA replication or repair on the damaged DNA. The gap filling involves polymerization through UV lesions (also termed bypass synthesis or error-prone repair) by DNA polymerase III. The primary source of mutations is the incorporation of incorrect nucleotides opposite lesions. UV mutagenesis is a genetically regulated process, and it requires the SOS-inducible proteins RecA, UmuD, and UmuC. It may represent a minor repair pathway or a genetic program to accelerate evolution of cells under environmental stress conditions.

Roles of Salmonella typhimurium umuDC and samAB in UV mutagenesis and UV sensitivity

Expression of the umuDC operon is required for UV mutagenesis and most chemical mutagenesis in Escherichia coli. The closely related species Salmonella typhimurium has two sets of umuDC-like operons; the samAB operon is located in a 60-MDa cryptic plasmid, while the S. typhimurium umuDC (umuDCST) operon resides in a chromosome. The roles of these two umuDC-like operons in UV mutagenesis and UV sensitivity of S. typhimurium were investigated. A pBR322-derived plasmid carrying the samAB operon more efficiently restored UV mutability to a umuD44 strain and a umuC122::Tn5 strain of E. coli than a plasmid carrying the umuDCST operon did. When the umuDCST operon was specifically deleted from the chromosome of S. typhimurium TA2659, the resulting strain was not UV mutable and was more sensitive to the killing effect of UV irradiation than the parent strain was. Curing of the 60-MDa cryptic plasmid carrying the samAB operon did not influence the UV mutability of strain TA2659 but did increase its resistance to UV killing. A pSC101-derived plasmid carrying the samAB operon did not restore UV mutability to a umuD44 strain of E. coli, whereas pBR322- or pBluescript-derived plasmids carrying the samAB operon efficiently did restore UV mutability. We concluded that the umuDCST operon plays a major role in UV mutagenesis in S. typhimurium and that the ability of the samAB operon to promote UV mutagenesis is strongly affected by gene dosage. Possible reasons for the poor ability of samAB to promote UV mutagenesis when it is present on low-copy-number plasmids are discussed.

Mutagenesis induced by bacterial UmuDC proteins and their plasmid homologues

The popular image of a world full of pollutants mutating DNA is only partly true since there are relatively few agents which can subtly and directly change base coding; for example, some alkylating agents alter guanine so that it pairs like adenine. Many more mutagens are less subtle and simply destroy coding altogether rather than changing it. Such mutagens include ultraviolet light, X-rays, DNA cross-linkers and other agents which make DNA breaks or large adducts. In Escherichia coli, mutagenesis by these agents occurs during a DNA repair process which increases cell survival but with an inherent possibility of changing the original sequence. Such mutagenic DNA repair is, in part, encoded by the E. coli umuDC operon. This article reviews the structure, function, regulation and evolution of the umuDC operon and similar genes found both in other species and on naturally occurring plasmids.

Escherichia coli umuDC mutants: DNA sequence alterations and UmuD cleavage

The products of the chromosomally encoded umuDC genes are directly required for mutagenesis in Escherichia coli. Strains with either umuD or umuC mutations are rendered phenotypically non-mutable. To ascertain the molecular basis of this non-mutability, we determined the DNA sequence alterations of seven chromosomal umuDC mutants. Six mutants (umuD1, umuD44, umuD77, umuC36, umuC25, and umuC104) were found to be single base-pair substitutions that resulted in missense mutations. The Tn5 transposon insertion mutation (umuC122) resulted in a missense mutation followed immediately by a termination codon, producing a truncated UmuC protein lacking 102 carboxyl-terminal amino acids. All of the mutations were found to reside in regions of the UmuD and UmuC proteins that share high homology with analogous proteins. Chemiluminescent immunoassays revealed that the umuD1, umuD44, and umuD77 mutations all resulted in a non-cleavable UmuD protein. Because UmuD cleavage is a prerequisite for mutagenesis, the lack of UmuD processing appears to be the molecular basis for the non-mutable phenotype in these strains. These studies re-emphasize the critical nature of the RecA-mediated cleavage of UmuD for inducible mutagenesis and provide insights into the functional domains of the UmuC protein.

Alleviation of EcoK DNA restriction in Escherichia coli and involvement of umuDC activity

The activity of the EcoK DNA restriction system of Escherichia coli reduces both the plating efficiency of unmodified phage lambda and the transforming ability of unmodified pBR322 plasmid DNA. However, restriction can be alleviated in wild-type cells, by UV irradiation and expression of the SOS response, so that 10(3)- to 10(4)-fold increases in phage growth and fourfold increases in plasmid transformation occurred with unmodified DNA. Restriction alleviation was found to be a transient effect because induced cells, which initially failed to restrict unmodified plasmid DNA, later restricted unmodified phage lambda. Although the SOS response was needed for restriction alleviation, constitutive SOS induction, elicited genetically with a recA730 mutation, did not alleviate restriction and UV irradiation was still needed. A hitherto unsuspected involvement of the umuDC operon in this alleviation of restriction is characterized and, by differential complementation, was separated from the better known role of umuDC in mutagenic DNA repair. The need for cleavage of UmuD for restriction alleviation was shown with plasmids encoding cleavable, cleaved, and non-cleavable forms of UmuD. However, UV irradiation was still needed even when cleaved UmuD was provided. The possibility that restriction alleviation occurs by a general inhibition of the EcoK restriction/modification complex was tested and discounted because modification of lambda was not reduced by UV irradiation. An alternative idea, that restriction activity was competitively reduced by an increase in EcoK modification, was also discounted by the lack of any increase in the modification of lambda Ral-, a naturally undermodified phage. Other possible mechanisms for restriction alleviation are discussed.