T4 and the rolling circle model of replication.

Abstract

The genetic map of T4 is circular (I), even though the DNA molecules of the infectious phage are linear: Map circularity results from the fact that, among the particles of a T4 population, the ends of the DNA molecules are randomly located in the map, rather than at a unique genetic site (2)1. Two genetic markers located near opposite ends of the genome in one individual phage particle would thus be closely linked in the large majority of the other members of the population. The genomes of T4 are furthermore known to be diploid for the terminal 1-2% of their length, carrying homologous genes at both the right and left ends of the DNA molecule (3). This circularly permuted terminal redundan­ cy appears to play an important role in genetic recombination. The non-encapsidated T4 DNA, vegetative-state DNA, within the infected bacterium consists in large part of concatemers (4) which are composed of two or more phage genomes in a linear continuum. It is thought that phage capsids (or their precursors) are loaded with DNA by filling them to capacity from such concatemers (headful hypothesis) (5). At least two ways have been suggested for originating such concatemers within the cell. One would consist of making many copies of the parental genomes which were injected into the host bacterium and forming concatemers by recombination among these daughter molecules. In the case of single infection such recombinations could occur in joining the right and left redundant ends of different copies, yielding a concatemer almost twice as long as the original parental DNA. Following multiple infection, which would generally provide parental molecules with ends in different genetic locations, concatemer formation could be achieved by recombination of the end of one