Hyperrecombination at a specific DNA sequence in pneumococcal transformation.

Abstract

In pneumococcal transformation, recombination frequency between point mutations is usually proportional to physical distances. We have identified an aberrant marker belonging to the amiA locus that appeared to markedly enhance recombination frequency when crossed with any other markers of this gene. This mutation results from the C-to-A transversion in the sequence A-T-T-C-A-T -+ A-T-T-A-A-T. This effect is especially apparent for short distances as small as 27 base pairs. The hyperrecombination does not require the wild-type function of the pneumococcal gene for an ATP-dependent DNase (which is homologous to the product of the Escherichia coli recBC genes) or of the hex genes, which correct certain mismatched bases in transformation. The hyperrecombination is affected by the presence of nearby mismatched bases that trigger an excision-repair system. It is proposed that the mutation that shows hyperrecombination is sometimes converted to the wild-type allele at the heteroduplex stage of transformation.General recombination involves the exchange of genetic information between homologous DNA segments. The frequency of recombination has been postulated to be approximately constant throughout homologous regions. However, this assumption is not always true, as has been shown by genetic studies. Many examples of map distortions have been reported. In fungi as well as other eukaryotes, the frequency of exchange between genes can be strongly affected by mutations (1). The nature of the genetic factors controlling the recombination events has remained elusive. A major step in the molecular understanding of one such factor has been the studies by Stahl and his collaborators of sites (Chi elements) that promote recombination in bacteriophage X (2). Such sites are also found in the Escherichia coli chromosome and may affect bacterial recombination (3). It was found that the enhancement of recombination due to Chi sites was asymmetric, and it required the wild-type activity of recBC genes. A specific octamer is present in all reported Chi elements (3). Recombination hotspots also have been observed in recombination between E. coli plasmids. These hotspots appear to be different from Chi elements in that they do not require the recBC pathway and they are active in either orientation (4). Moreover, Lieb identified mutations that enhance recombinations in bacteriophage crosses (5). Such mutations are particularly susceptible to mismatch correction by short patch repair and are quite different from the Chi elements. In studies of marker effects in bacteriophage T4 recombination, short patch repair has also been reported (6,7). Therefore, it appears that there are a variety of factors that control recombination, which may be more or less specific to the organism under investigation. Moreover, the mechanisms of enhanced recombination could be better understood if genes that promote such an effect were known.The subject of this report is the description of a mutation belonging to the amiA locus of Streptococcus pneumoniae t...

Very short patch repair of T:G mismatches in vivo: importance of context and accessory proteins

“…This indicates that context is an important factor in MutY-mediated short patch repair, as suggested by others (33,40). Sequencespecific short patch mismatch repair also occurs in Streptococcus pneumoniae during transformation (25). A C-to-A transversion in the sequence 5ЈATTCAT is corrected when the A is mispaired with G. Base substitutions at all positions reduced correction of A:G, and a T-to-C transition in position 6 abolished mismatch repair (17).…”

Hyperrecombination in Streptococcus pneumoniae Depends on an Atypical mutY Homologue

“…With regard to this rule of proportionality, ami36 generates an abnormally high frequency of ami ϩ transformants. For example, crosses between ami36 and ami6, 27 bp apart, produce 20 to 25% ami ϩ bacteria while about 1% is predicted by the distance (24). The pneumococcal strains 553 (hexB ami36 mutY ϩ ) and 553T51 (hexB ami36 mutY::pRT51), were transformed with a chromosomal DNA carrying the mutations ami6 and str41 in order to test whether the hyperrecombination induced by ami36 was affected by the inactivation of mutY.…”

“…The pneumococcal strains 553 (hexB ami36 mutY ϩ ) and 553T51 (hexB ami36 mutY::pRT51), were transformed with a chromosomal DNA carrying the mutations ami6 and str41 in order to test whether the hyperrecombination induced by ami36 was affected by the inactivation of mutY. Hex-deficient strains were used as the recipients to avoid interferences between the Hex system and the hyperrecombination process (24). Hyperrecombination was abolished in the strain 553T51 ( Table 2).…”

“…However, the genetic control of this repair was still unknown. As the sequence flanking ami36 looked important for this hyperrecombination (12,24), it was actually unclear whether this pathway was more related to MutY, which repairs A/G to CG, or to the VSP pathway, which repairs G/T to GC in defined sequence environments (reviewed in reference 25). As we show here that the disruption of a pneumococcal mutY homologue abolishes hyperrecombination, we can assume that the main protein controlling the A/G-to-CG repair involved in this hyperrecombination is a MutY-like adenine glycosylase.…”

“…The existence of such a system was signalled by the hyperrecombination-i.e., the abnormally high frequency of wild-type transformants-shown by the mutation ami36 when involved in two point crosses (24). The mutation ami36 results from a CG-to-AT transversion and, consequently, forms upon transformation the mismatches A 36 /G ϩ and C ϩ /T 36 .…”