The hyperthermophilic bacterium Thermotoga maritime has two different classes of family C DNA polymerases: evolutionary implications

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Abstract
Bacterial DNA polymerase III (family C DNA polymerase), the principal chromosomal replicative enzyme, is known to occur in at least three distinct forms which have provisionally been classified as class I (Escherichia coli DNA pol C-type), class II (Bacillus subtilis DNA pol C-type) and class III (cyanobacteria DNA pol C-type). We have identified two family C DNA polymerase sequences in the hyperthermophilic bacterium Thermotoga maritima. One DNA polymerase consisting of 842 amino acid residues and having a molecular weight of 97 213 belongs to class I. The other one, consisting of 1367 amino acid residues and having a molecular weight of 155 361, is a member of class II. Comparative sequence analyses suggest that the class II DNA polymerase is the principal DNA replicative enzyme of the microbe and that the class I DNA polymerase may be functionally inactive. A phylogenetic analysis using the class II enzyme indicates that T.maritima is closely related to the low G+C Gram-positive bacteria, in particular to Clostridium acetobutylicum, and mycoplasmas. These results are in conflict with 16S rRNAbased phylogenies, which placed T.maritima as one of the deepest branches of the bacterial tree.