Bacteriophage λ p gene shows host killing which is not dependent on λ DNA replication

Abstract

Bacteriophage λ having a mutation replacing glycine by glutamic acid at the 48th codon of cro, kills the host under N^- conditions; we call this the hk mutation. In λN^-N^-cl^-hk phage-infected bacteria, the late gene R i expressed to a significant level, phage DNA synthesis occurs with better efficiency, and the Cro activity is around 20% less, all compared to those in λN^-N^-cl^-hk⁺-infected bacteria. Segments of λ DNA from the left of pR to the right of tR2, carrying cro, cll, O,P, and the genes of the nin5 region from the above hk and hk⁺ phages, were cloned in pBR322. Studies with these plasmids and their derivatives having one or more of the λ genes deleted indicate that the hk mutation is lethal only when a functional P gene is also present. When expression of P from pR is elevated, due to the deletion of tR1, host killing also occurs without the hk mutation. We conclude that the higher levels of P protein, produced either (1) when cro has the hk mutation or (2) when tR1 is deleted, are lethal to the host. We also show that due to the hk mutation, the Cro protein becomes partially defective in its negative regulation at pR, resulting in the expression of P to a lethal level even in the absence of N protein-mediated antitermination. This P protein-induced host killing depends neither on λ DNA replication nor on any other gene functions of the phage.