MULTIPLE INITIATION MECHANISMS ADAPT PHAGE-T4 DNA-REPLICATION TO PHYSIOLOGICAL-CHANGES DURING T4S DEVELOPMENT

We summarize the evidence for multiple pathways to initiate phage T4 D NA replication. In any infecting chromosome, leading DNA strands can b e primed from pre-replicative transcripts, independent of primase acti vity, at one of several origins. Within each origin region, there are multiple RNA-DNA transition sites. However, the priming potential at e ach single site is very low. Our results suggest that origin transcrip ts can become primers for leading strand DNA synthesis without being p rocessed, but that a promoter-proximal segment of each origin transcri pt plays an important structural role, as a proposed wedge, in the tra nsition from RNA to DNA synthesis. Two recombination-dependent pathway s render subsequent phage T4 DNA replication independent of transcript ion. The first of these requires proteins that are synthesized during the pre-replicative phase of infection. It is active as soon as the fi rst growing points, initiated at origins, have reached a chromosomal e nd. The other one requires at least one late protein: endonuclease VII , a resolvase that cuts recombinational junctions. The latter pathway can bypass primase deficiencies by allowing retrograde DNA synthesis w ithout Okazaki pieces. We discuss the integration of these multiple an d redundant pathways into the developmental program of T4. Competition between these initiation mechanisms and with other DNA transactions a llows for integration of replication controls with transcription, reco mbination and packaging of the DNA.