STRUCTURAL AND FUNCTIONAL ANALYSIS OF DNA REPLICATION INITIATION PROTEINS FROM THE ARCHAEON METHANOTHERMOBACTER THERMAUTOTROPHICUS
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The faithful duplication of the chromosome requires the combined efforts of numerous proteins. Cdc6 and MCM are two such proteins involved in the initiation of DNA replication. The genome of the euryarchaeon Methanothermobacter thermautotrophicus contains one MCM and two Cdc6 homologues (Cdc6-1 and -2). While MCM is the replicative helicase that unwind the duplex DNA to provide single-stranded DNA substrate for the replicative polymerases, the Cdc6 proteins are presumed to function in origin recognition and helicase assembly at the origin. This thesis elucidates the structure, function and regulation of these archaeal initiation proteins. The M. thermautotrophicus MCM helicase is a dumb-bell shaped double hexamer. Each monomer can be divided into two portions. The C-terminal catalytic region contains the ATP binding and hydrolysis sites essential for helicase activity. This thesis concentrates its efforts to determine the functional role of the N-terminal region. Using a variety of biochemical approaches it was found that the N-terminal portion of MCM is involved in hexamer/dodecamer formation. The study also identified two structural features at the N-terminus, the zinc- and the beta-finger motifs, essential for DNA binding, which in turn is essential for helicase activity. In addition, the N-terminal portion of MCM interacts with both Cdc6 proteins. The role of the Cdc6-1 and -2 proteins in origin recognition and helicase loading was also elucidated. The results presented in this thesis show that Cdc6-1 has binding specificity to origin DNA sequences suggesting a role for the protein in origin recognition. While both Cdc6 proteins interact with the MCM helicase, Cdc6-2 exhibited tighter binding compared to Cdc6-1 suggesting a role for Cdc6-2 in helicase loading. Summarizing the observations of this study, a model for the replication initiation process in M. thermautotrophicus has been proposed, outlining separate role for the two Cdc6 proteins, Cdc6-1 in origin recognition and Cdc6-2 in MCM helicase assembly at the origin.