Putative MutS2 Homologs in Algae: More Goods in Shopping Bag?

MutS2 proteins are presumably involved in either control of recombination or translation quality control in bacteria. MutS2 homologs have been found in plants and some algae; however, their actual diversity in eukaryotes remains unknown. We found putative MutS2 homologs in various species of photosynthetic eukaryotes and performed a detailed analysis of the revealed amino acid sequences. Three groups of homologs were distinguished depending on their domain composition: MutS2 homologs with full set of specific domains, MutS2-like sequences without endonuclease Smr domain, and MutS2-like homologs lacking Smr and clamp in domain IV, the extreme form of which are proteins with only a complete ATPase domain. We clarified the information about amino acid composition and set of specific motifs in the conserved domains in MutS2 and MutS2-like sequences. The models of the predicted tertiary structure were obtained for each group of homologs. The phylogenetic analysis demonstrated that all eukaryotic sequences split into two large groups. The first group included homologs belonging to species of Archaeplastida and a subset of haptophyte homologs, while the second-sequences of organisms from CASH groups (cryptophytes, alveolates, stramenopiles, haptophytes) and chlorarachniophytes. The cyanobacterial MutS2 clustered together with the first group, and proteins belonging to Deltaproteobacteria (orders Myxococcales and Bradymonadales) showed phylogenetic affinity to the CASH-including group with strong support. The observed tree pattern did not support a clear differentiation of eukaryotes into lineages with red and green algae-derived plastids. The results are discussed in the context of current conceptions of serial endosymbioses and genetic mosaicism in algae with complex plastids.