A NEW GENE OF BACTERIOPHAGE-P4 THAT CONTROLS DNA-REPLICATION

Bacteriophage P4 replication may result in either a lytic cycle or pla smid maintenance, depending on the presence or absence, respectively, of helper phage P2 genome. Bacteriophage P4 DNA replication depends on the product of gene alpha, which has origin recognition, primase, and helicase activities. An open reading frame with the coding capacity f or a protein of 106 amino acids (orf106) is located upstream of the al pha gene. Genes orf106 and alpha are transcriptionally coregulated. Th ree amber mutations and an internal deletion (del51) were introduced i nto orf106. All of the amber mutations exhibited a polar effect on tra nscription of the downstream alpha gene. The P4 del51 mutant was sligh tly defective in lytic growth and could not be propagated in the plasm id state. In this latter condition, P4 DNA overreplication was observe d. Overexpression of Orf106 severely inhibited P4 DNA replication, pre venting P4 lytic growth and plasmid maintenance. The inhibitory effect of Orf106 on P4 replication was not observed when both orf106 and alp ha were overexpressed. We suggest that orf106 is involved in P4 replic ation and that a balanced expression of orf106 relative to alpha may b e necessary for proper P4 DNA replication. In particular, orf106 appea rs to be essential for the control of P4 genome replication in the pla smid state. We propose that orf106 be named cnr, for copy number regul ation.