The minichromosome maintenance (MCM) complex is thought to function as the replicative helicase in archaea and eukaryotes. In eukaryotes, this complex is an assembly of six different but related polypeptides (MCM2-7) but, in most archaea, one MCM protein assembles to form a homohexameric complex. Atypically, the Thermococcus kodakarensis genome encodes three archaeal MCM homologs, here designated MCM1-3, although MCM1 and MCM2 are unusual in having long and unique N-terminal extensions. The results reported establish that MCM2 and MCM3 assemble into homohexamers and exhibit DNA binding, helicase and ATPase activities in vitro typical of archaeal MCMs. In contrast, MCM1 does not form homohexamers and although MCM1 binds DNA and has ATPase activity, it has only minimal helicase activity in vitro. Removal of the N-terminal extension had no detectable effects on MCM1 but increased the helicase activity of MCM2. A T. kodakarensis strain with the genes TK0096 (MCM1) and TK1361 (MCM2) deleted has been constructed that exhibits no detectable defects in growth or viability, but all attempts to delete TK1620 (MCM3) have been unsuccessful arguing that that MCM3 is essential and is likely the replicative helicase in T. kodakarensis. The origins and possible function(s) of the three MCM proteins are discussed.
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