GENE DUPLICATIONS IN EVOLUTION OF ARCHAEAL FAMILY-B DNA-POLYMERASES

All archaeal DNA-dependent DNA polymerases sequenced to date are homol ogous to family B DNA polymerases from eukaryotes and eubacteria, Pres ently, representatives of the euryarchaeote division of archaea appear to have a single family B DNA polymerase, whereas two crenarchaeotes, Pyrodictium occultum and Sulfolobus solfataricus, each possess two fa mily B DNA polymerases. We have found the gene for yet a third family B DNA polymerase, designated B3, in the crenarchaeote S. solfataricus P2. The encoded protein is highly divergent at the amino acid level fr om the previously characterized family B polymerases in S. solfataricu s P2 and contains a number of nonconserved amino acid substitutions in catalytic domains, We have cloned and sequenced the ortholog of this gene from the closely related Sulfolobus shibatae. It is also highly d ivergent from other archaeal family B DNA polymerases and, surprisingl y, from the S. solfataricus B3 ortholog, Phylogenetic analysis using a ll available archaeal family B DNA polymerases suggests that the S. so lfataricus P2 B3 and S. shibatae B3 paralogs are related to one of the two DNA polymerases of P. occultum. These sequences are members of a group which includes all euryarchaeote family B homologs, while the re maining crenarchaeote sequences form another distinct group, Archaeal family B DNA polymerases together constitute a monophyletic subfamily whose evolution has been characterized by a number of gene duplication events.