Movement and site selection for priming by the primosome in phage phi X174 DNA replication.

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Abstract
The Escherichia coli priming system used for initiation of DNA chains on phage .vphi.X174 single-stranded DNA is a multiprotein unit called the primosome. Assembled with participation of 7 prepriming proteins and primase at a unique place on the .vphi.X174DNA template, the primosome is bound tightly to the DNA, yet moves rapidly and unidirectionally opposite to primer and DNA chain synthesis. Contributions of protein n'' and dnaB protein, 2 components of the primosome, to movement and site selection for priming were studied. Figuratively, the primosome can be likened to a locomotive that depends on protein n'' as its engine and dnaB protein as the engineer. Protein n'', a DNA-dependent ATPase (dATPase) appears to use the energy of hydrolysis of the nucleoside triphosphate for processive translocation of the primosome. The dnaB protein, a DNA-dependent ribonucleoside triphosphatase, depends on allosteric effects of a nucleoside triphosphate to induce changes in the structure of the single-stranded DNA at preferred sequences that enable primase to synthesize a short primer for initiation of DNA synthesis. These primosome properties have important implications for the progress of the replication fork of the E. coli chromosome.

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