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Index >> Bacteriophages - Part One >> Presynthesis Stage of øX174 Phage

Presynthesis Stage of øX174 Phage

	Presynthesis Stage of øX174 Phage - The presynthesis reaction requires the following components (Wickner and Hurwitz, 1974, 1975) : ØX174 ssDNA, dnaB protein, dnaC protein, unwinding protein. protein factors X, Y and Z and A TP. dnaB mutants arc defective in elongation and dnaC mutants in initiation. dnaA proteins are not required in this reaction.Weiner, McMacken and Kornberg (1976) have obtained similar results.
According to them the presynthetic reaction requires ssDNA, dnaB protein, dnaC protein, unwinding protein, A TP and two protein factors, i and n. In M13 and G4 only the unwinding protein is required in the presynthesis stage. Proteins i and n, the unwinding protein and the dnaB and dnaC proteins form an intermediate that, serves as a substrate for the dnaG protein (Weiner ct. al, 1976)
The ssDNA circle is extended by an unwinding protein, leaving a hairpin duplex. There are about 800 molecules of this protein per cell, corresponding to about 130 copies at each replication fork. The protein appears to have a structural role in establishing ssDNA in a template state it binds to ssDNA but not to dsDNA.
The unwinding protein masks ssDNA in all but the 'promoter' region (duplex hairpin region). It might thus permit initiation of DNA synthesis only at the origin within the dsDNA region. The action of RNA polymerase takes place in this region.

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