| Abstract
| - Gene A protein of bacteriophage ΦX174 plays a role as a site-specific endo-nuclease in the initiation and termination of ΦX rolling circle DNA replication. To clarify the sequence requirements of this protein we have studied the cleavage of single-stranded restriction fragments from ΦX and G4 viral DNAs using purified gene A protein. The results show that in both viral DNAs cleavage occurs at the origin and at one additional site which shows striking sequence homology with the origin region. During rolling circle replication the single-stranded viral DNA tail is covered with single-stranded DNA binding (SSB) protein. Therefore, we have also studied the effect of SSB on ΦX gene A protein cleavage. In these conditions only single-stranded fragments containing the complete or almost complete origin region of 30 bases are cleaved, whereas cleavage at the additional sites of ΦX or G4 viral DNAs does not occur. A model for termination of rolling circle replication which is based on these findings is presented. Finally, we present evidence that the second product of gene A, the A* protein, cleaves ΦX viral DNA at the additional cleavage site in the presence of SSB, not only in vitro but also in vivo. The functional significance of this cleavage in vivo is discussed.
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