DNA maturation by the “headful” mode in bacteriophage T1

Abstract

The finding of a limited set of cyclic permutations of the nucleotide sequence in bacteriophage T1 DNA ( Gill & MacHattie, 1976 ) has suggested that this DNA is matured by a “headful” mechanism, as opposed to a “unique” mechanism directed toward sequence-specific sites for the location of the molecular end-points. The limited range of permutations in this case would, in contrast to the more general headful model, require sequence recognition at one site, from which a limited number of headfuls would be “measured off” and cut for packaging by a nonsquence-specific mechanism. In contrast to the unique mode of maturation, this model would be expected to produce mature molecules of slightly heterogeneous molecular weight, as has been observed in other headful packaging systems ( Kim & Davidson, 1974 ). Experiments are reported which confirm this model of headful maturation in T1, by measurements of the quantitative variations in terminal repetition length which accompany changes in genome or specific sequence length in a group of T1-related hybrid phages produced by crosses with the Shigella phage D20. The D20 nucleotide sequence is shown to exhibit at least 11 discrete regions of non-homology with that of T1. Different segments of the D20 sequence present in the different hybrids confer measurable alterations in genome length, which are shown to result in equal but opposite alterations in terminal repetition length, as predicted by the headful packaging model. The molecular weight heterogeneity in T1 DNA, previously observed as a variability in terminal repetition length in individual molecules ( MacHattie et al. , 1972 ), is confirmed and more accurately assessed by measurements of the relative band widths of T1 and lambda phage particles in equilibrium density gradients. From the positions of D20 segments presumably carrying the equivalents of T1 genes 1 and 2 in the hybrid phage DNAs, the orientation of the T1 genetic map is established with respect to the unique direction of in vitro DNA extrusion from T1 particles previously reported ( Gill & MacHattie, 1975 ). The proposed recognition site for packaging initiation is accurately placed within the T1: D20 non-homology map, and shown to lie approximately between the DNA synthesis genes 1 and 2 .