Functions of the Mismatch Repair Gene mutS from Acinetobacter sp. Strain ADP1

Abstract

The genus Acinetobacter encompasses a heterogeneous group of bacteria that are ubiquitous in the natural environment due in part to their ability to adapt genetically to novel challenges. Acinetobacter sp. strain ADP1 (also known as strain BD413) is naturally transformable and takes up DNA from any source. Donor DNA can be integrated into the chromosome by recombination provided it possesses sufficient levels of nucleotide sequence identity to the recipient's DNA. In other bacteria, the requirement for sequence identity during recombination is partly due to the actions of the mismatch repair system, a key component of which, MutS, recognizes mismatched bases in heteroduplex DNA and, along with MutL, blocks strand exchange. We have cloned mutS from strain ADP1 and examined its roles in preventing recombination between divergent DNA and in the repair of spontaneous replication errors. Inactivation of mutS resulted in 3-to 17-fold increases in transformation efficiencies with donor sequences that were 8 to 20% divergent relative to the strain ADP1. Strains lacking MutS exhibited increased spontaneous mutation frequencies, and reversion assays demonstrated that MutS preferentially recognized transition mismatches while having little effect on the repair of transversion mismatches. Inactivation of mutS also abolished the marker-specific variations in transforming efficiency seen in mutS ؉ recipients where transition and frameshift alleles transformed at eightfold lower frequencies than transversions or large deletions. Comparison of the MutS homologs from five individual Acinetobacter strains with those of other gram-negative bacteria revealed that a number of unique indels are conserved among the Acinetobacter amino acid sequences.

Impact of mutS Inactivation on Foreign DNA Acquisition by Natural Transformation in Pseudomonas stutzeri

“…In transformation of Acinetobacter sp. strain BD4 the extent of sexual isolation conferred by MMR is about 22%, as estimated from the data presented by Young and Ornston (89). The observed strong reduction in interspecific transformation of cells with multiple copies of the mutS ϩ gene also supports the view that normally MMR does not provide a strong barrier to genetic exchange in P. stutzeri.…”

“…These observations for P. stutzeri are different from the generally lower level of repair of transversion mismatches during natural transformation of S. pneumoniae (11,12) and Acinetobacter sp. strain BD4 (89). Transversion allele 4 (Table 3), which can result in CC and GG mismatches in the transformants, showed only a fourfoldhigher transformation frequency in the mutS mutant than in the mutS ϩ strain.…”

Rapid Evolution of Diminished Transformability in Acinetobacter baylyi

“…The ⌬mutS strain demonstrated ϳ100-fold increased mutation frequency, as measured by the fraction of the population that spontaneously acquired rifampin resistance in 20 generations (data not shown). This is a mutation frequency comparable to what has previously been measured for disrupted mutS genes in A. baylyi (44). The growth rate of the ancestral ⌬mutS lineage did not differ significantly from that of the wild-type ancestor.…”

Growth phase-specific evolutionary benefits of natural transformation in Acinetobacter baylyi

“…Two groups contained transformation-proficient populations (designated Com+) and the third group comprised transformation-deficient populations (designated Com − ). To explore the potential benefits of heterologous DNA in the evolving populations, a DNA mix from Spitzbergen soil, pig feces, Acinetobacter baumannii AZR3410 (Ray et al, 2009) and Acinetobacter johnsonii LUH540 (Young and Ornston, 2001) was added to one of the transformation-proficient groups during the serial transfers (see Supplementary Information for description; group Com+DNA). The other transformation-proficient group (Com+) could only acquire DNA from within the population.…”