Hyperrecombination in Streptococcus pneumoniae Depends on an Atypical mutY Homologue

Abstract

The unusual behavior of the mutation ami36, which generates hyperrecombination in two point crosses, was previously attributed to a localized conversion process changing A/G mispairs into CG pairs. Although the mechanism was found to be dependent on the DNA polymerase I, the specific function responsible for this correction was still unknown. Analysis of the pneumococcal genome sequence has revealed the presence of an open reading frame homologous to the gene mutY of Escherichia coli. The gene mutY encodes an adenine glycosylase active on A/G and A/7,8-dihydro-8-oxoguanine (8-OxoG) mismatches, inducing their repair to CG and C/8-OxoG, respectively. Here we report that disrupting the pneumococcal mutY homologue abolishes the hyperrecombination induced by ami36 and leads to a mutator phenotype specifically enhancing AT-to-CG transversions. The deduced amino acid sequence of the pneumococcal MutY protein reveals the absence of four cysteines, highly conserved in the endonuclease III/MutY glycosylase family, which ligate a [4Fe-4S] 2؉ cluster. The actual function of this cluster is still intriguing, inasmuch as we show that the pneumococcal gene complements a mutY strain of E. coli.In transformation of Streptococcus pneumoniae, doublestranded DNA binds to the membrane and is randomly cleaved (see reference 23 for a review). Then, single-stranded segments enter the cell from a 3Ј end while the complementary strands are degraded to oligonucleotides with the opposite polarity (32). About half of the entering segments integrate into the chromosome by homologous recombination (21). The recombination process is RecA dependent (37), exchanges strands from 5Ј to 3Ј relative to the donor (43), and forms a donorrecipient structure which is heteroduplex when the donor and the recipient sequences are not identical (11). Both strands of the donor DNA have the same probability of entering a cell so that two complementary heteroduplexes are generated in equal frequency among the recipient bacteria (7). Pneumococcal transformation therefore allows study of the in vivo processing of heteroduplexes such as base-base mismatches. In particular, the variability observed in the transformation efficiencies of point mutations led to the discovery of a mismatch repair system (10,20,55). This system, called Hex, recognizes the different mismatches with varying efficiencies and induces the complete excision of the donor strand (31). Base mismatches are ranked as a function of decreasing repair efficiency by Hex as follows: (6). The deletions and the additions of one or two nucleotides lead to mismatches that are very efficiently recognized by Hex (13,14). The heterologies longer than 2 bases lead to heteroduplexes which are poorly recognized by Hex, and for those longer than 5 bases, there is no repair at all by Hex (13,22) nor by any bacterial repair or conversion system (42).A Hex-independent repair, specific for A/G mismatches, was found in S. pneumoniae. The existence of such a system was signalled by the hyperrecombination-i.e., the abn...

Characterization of In Vitro Biofilm-Associated Pneumococcal Phase Variants of a Clinically Relevant Serotype 3 Clone

“…Since phase variants can arise through point mutations (12), it is plausible that phase variation is partially regulated by mismatch repair genes. In S. pneumoniae, the mismatch repair genes include the Hex genes (11,15) and the Hex-independent genes, the mutY and mutX genes (8,25,30), and the pms gene (22). The rough phase variants with deletions, which have not previously been described in a biofilm model, did not appear to arise through recombinational events of nontandem repeats (28).…”

Competence Repression under Oxygen Limitation through the Two-Component MicAB Signal-Transducing System in Streptococcus pneumoniae and Involvement of the PAS Domain of MicB

“…ORFs flanking micB were designated micA and orfC. The micAB genes are located close to the antimutator, mutY (2,16,24) (Fig. 1A).…”

“…In B. subtilis, insertion mutation in yycJ is silent (9), and this issue has not been addressed for the yycJ of S. aureus (17). MutY of S. pneumoniae was recently identified by computer-aided studies and by complementation of a mutY mutant of E. coli (24). This antimutator gene in E. coli and S. pneumoniae encodes an adenine glycosylase specific for A/G miss-match pairs.…”

“…These (4Fe-4S) 2ϩ clusters have been extensively described for proteins that sense the redox status of the cells (27). MutY of S. pneumoniae has no FCL domain (24) and lies near micAB, a signaling system involved in the cellular response to oxygen.…”

Antimutator Role of DNA Glycosylase MutY in Pathogenic Neisseria Species

“…An evolutionary analysis of the HhH superfamily of DNA repair glycosylases performed by Denver et al shows that MutY is present in most bacteria, many eukaryotes, and nearly 50% of the archaea investigated (9). The crystal structure of E. coli MutY has been solved (15,19), and the gene has been cloned and characterized in a range of species, including mammals (23,24,28,43). However, none of these species are directly comparable with MC.…”