Primary structure of the replication initiation protein of plasmid R6K

The cistron of the replication initiation protein of plasmid R6K has been cloned into the single-strand DNA vectors M13mp8 and M13mp9 and its complete nucleotide sequence has been determined. The amino acid sequence of the initiator protein as predicted from its nucleotide sequence shows that the protein is lysine rich and weakly basic and has a molecular weight of 35,000, which is in close agreement with that estimated from the mobility in NaDodSO4/acrylamide gels. The secondary structure of the protein, approximately by the probabilistic methods of Chou and Fasman [Chou, P. & Fasman, G. (1978) Adv. Enzymol. 47, 45-148], suggests an NH2-terminal domain of primarily positively charged alpha-helical structure, a core region of interspersed short stretches of random coils and beta-sheets and -turns, and a COOH-terminal domain of alpha-helix.