DNA mismatch repair (MMR) systems can be classified as either MutH-dependent or 
MutH-independent. In bacteria, extensive studies have been conducted with the 
MutH-dependent MMR in Escherichia coli and its close relatives. The picture of 
MutH-independent MMR in other bacteria is less clear, as MMR components other 
than MutS and MutL have not been identified in the majority of bacteria. 
Bacillus anthracis is one of the MutH-less Gram(+) bacteria in the phylum of 
Firmicutes. We used papillation as a tool to search for B. anthracis new mutator 
strains and identified a spontaneous mutator that carries a minitransposon 
insertion in the BAS4289 locus. The mutational frequency and specificity 
exhibited in this mutant were comparable to that of MMR-deficient strains with 
knockouts of mutL or mutS. It retained a similar UV sensitivity profile as that 
of the wild type. BAS4289 encodes a putative DNA helicase RecD2 that shares 30% 
sequence identity with Deinococcus radiodurans RecD2, a well characterized 
superfamily 1B helicase whose homologs are widely present in Firmicutes complete 
genomes. We demonstrated that the N-terminal region of RecD2, a unique sequence 
extension used to distinguish RecD2 from RecD1, was important for B. anthracis 
RecD2, as mutations in the N-terminal conserved motifs affected its DNA repair 
function. This is the first report of a RecD2 helicase being associated with 
MMR. RecD2 and our recently described YycJ protein are likely to be two 
additional components in the B. anthracis MutH-independent MMR system.