Phage-coded protein prevents restriction of unmodified progeny T4 DNA

Abstract

HOST-controlled restriction modification systems usually prevent the survival of unmodified genomes in bacterial cells^1–4. Phage DNA can be modified by methylation (λ, and f1)^5–7 or by glucosylation (T-even phages³). Modification of the λ genome is mediated by a host methylase⁸ whereas modification of the T4 genome is by phage coded α- and β-glucosyl transferases⁹. The restriction systems of E. coli (rgl) acting on non-glucosylated (glu⁻) T-even phages are called r₆ and r_2,4 (ref. 10): r₆ restricts all glu⁻ T-even phages, whereas r_2,4 restricts only T2 and T4 glu⁻ phages. Hattman and Fukasawa¹¹ showed that when glu⁺ T4 DNA was injected into an rgl⁺ uridine diphosphoglucose (UDPG) pyrophosphorylaseless mutant, the progeny glu⁻ DNA replicated and was matured into virions which they called T^*. To explain why the replicating glu⁻ progeny DNA is not restricted in the rgl⁺ host while the entering T^* virion DNA is, Fukasawa¹² postulated that the rgl system is localised in the membrane. Incoming glu⁻ parental DNA would be restricted during passage through the membrane. Incoming glu⁺ parental DNA (which is not restricted) would be replicated as glu⁻ DNA in the absence of UDPG. Since the replicating DNA is not subject to the restriction system localised in the membrane it should not be restricted. Not surprisingly, r₆ was shown to be localised in the membrane^13,14. There is, however, good evidence that r_2,4 can restrict replicating DNA^15,16 and is, at least in that sense, cytoplasmic. Thus, the question why r_2,4 does not restrict unmodified progeny DNA is still unresolved. We report here that the phage specifies a protein which prevents the action of r_2,4. The anti-r_2,4 activity specified by the modified parental genome prevents restriction of the unmodified progeny DNA by r_2,4.