Initiation of DNA replication by the Escherichia coli dnaG protein: evidence that tertiary structure is involved.

The dnaG protein of Escherichia coli initiates DNA replication by synthesizing primer oligonucleotides for elongation by DNA polymerae. The experiments reported here probe the nature of the nucleic acid element recognized by the dnaG protein. Three well-separated groups of nucleotides within the negative-strand origin of the single-stranded phage phi K are protected by the dnaG protein against nuclease digestion. DNA as far as 115 bases from the start site of primer synthesis is involved in binding of the dnaG protein to the replication origin. One molecule of dnaG protein could protect all of these nucleotides if the DNA were folded into a higher-order tertiary structure. Protection of the phi K origin by dnaG protein requires DNA binding protein, and it does not occur if the group of protected nucleotides most distant from the start site is removed from the template. There is no binding of dnaG protein to the complementary strand of the phi K origin-region DNA. The observed protection of the positive strand is due to a functional nucleic acid-protein complex.