Bacteriophage T4 gene 41 helicase and gene 59 helicase-loading protein: A versatile couple with roles in replication and recombination

Bacteriophage T4 uses two modes of replication initiation: origin-dependent replication early in infection and recombination-dependent replication at later times. The same relatively simple complex of T4 replication proteins is responsible for both modes of DNA synthesis. Thus the mechanism for load ing the T4 41 helicase must be versatile enough to allow it to be loaded on 9 loops created by transcription at several origins, on D loops created by recombination, and on stalled replication forks. T4 59 helicase-loading pr otein is a small, basic, almost completely alpha -helical protein whose N-t erminal domain has structural similarity to high mobility group family prot eins. In this paper we review recent evidence that 59 protein recognizes sp ecific structures rather than specific sequences. It binds and loads the he licase on replication forks and on three- and four-stranded (Holliday junct ion) recombination structures, without sequence specificity. We summarize o ur experiments showing that purified T4 enzymes catalyze complete unidirect ional replication of a plasmid containing the T4 ori(uvsY) origin, with a p reformed 9 loop at the position of the 9 loop identified at this origin in vivo. This replication depends on the 41 helicase and is strongly stimulate d by 59 protein. Moreover, the helicase-loading protein helps to coordinate leading and lagging strand synthesis by blocking replication on the ori(uv sY) R loop plasmid until the helicase is loaded. The T4 enzymes also can re plicate plasmids with 9 loops that do not have a T4 origin sequence, but on ly if the 9 loops are within an easily unwound DNA sequence.