Virulence as a consequence of genome instability of a novel temperate bacteriophage, ΦRsG1, of Rhodobacter sphaeroides Y

Abstract

A novel temperate bacteriophage, designated ΦRsG1, was isolated from Rhodobacter sphaeroides Y (previously designated Rhodopseudomonas sphaeroides) following exposure to mitomycin C. The phage morphology, as revealed from electron microscopy, showed a hexagonal head (90 by 46.5 nm) connected with a tail (116 by 9.4 nm), to which a collar was proximally attached. A morphologically similar phage was also produced by spontaneous lysis of the cells. While ΦRsG1 did not grow on any other bacterial strain tested, spontaneously produced phage particles propagated (and formed plaques) on R. sphaeroides Y still carrying ΦRsG1 in the prophage state. The genome of ΦRsG1 consisted of double stranded linear DNA with cohesive ends and a GC-content of 71.8 mol%. The DNA molecules formed circles in vitro with a mean contour length of 17.18±0.4 μm, which corresponds to a size of 49 kbase pairs (kb). On the other hand, DNA extracted from the virulent phage particles was heterogeneous and consisted of two DNA species of different size, occurring in a ratio of about 1:1. These molecules also circularized having contour lengths of 17.18±0.4 μm and 14.02±0.41 μm corresponding to 49 and 40 kb, respectively. Restriction digest analysis of the two DNA species and DNA from ΦRsG1 indicated that they are similar, and allowed the indentification of an 11.5 kb EcoRI fragment that carries the cohesive ends. Because DNA from ΦRsG1 and the 49 kb DNA of the virulent phage particles were indistinguishable with the criteria applied, it is suggested that phage particles containing the 40 kb DNA represent the virulent type of phage, termed ΦRsG1.1.