Complementation assay of primer protein: Gene expression systems of plasmid vectors support the infection of suppressor sensitive mutant phages M2 and φ29

Two different expression systems of genes of primer proteins (pE for phage M2, and p3 for φ29) were constructed to study the protein primed DNA replication of Bacillus phages M2 and φ29. In one system, expression of the genes was under the control of the inducible spac promoter, whereas in the other system, the expression was under the control of the constitutive promoter in plasmid pUB110. Complementation tests in vivo were performed between the primer proteins expressed by these systems and mutant phages having suppressor sensitive mutations in the genes of the primer proteins. The phages M2 susE and φ29 sus3 were complemented by pE and p3 expressed by the systems, respectively. However, the complementation and apparent phage DNA synthesis were not detected in the combinations between susE of phage M2 and p3 of phage φ29, and vice versa. Although pE and p3 proteins exhibited structurally and functionally similar characteristics, these proteins showed species specificity in the protein primed DNA replication of bacteriophages M2 and φ29.