Host specificity of DNA produced by Escherichia coli. XII. The two restriction and modification systems of strain 15T-

Summary E. coli 15T^- carries two distinct sets of DNA restriction and modification activities. The genetic information for system A is contained in the bacterial chromosome and linked to the thr region. This fact suggests host specificity A to be related to those of strains K and B. The genes controlling system 15 are on a plasmid which is related to phage Pl: it competes with Pl for stable inheritance in the carried state and it genetically recombines with Pl. This recombination may produce plasmid genomes with newly assorted characters (see Table 3). One of them is an active, Pl-like prophage with the 15-specific instead of the parental Pl-specific restriction and modification characters. Superinfection of 15T^- with Pl may also result in curing of the bacteria from the restriction plasmid.Bot A- and 15-specific restrictions and modifications act on bacterial DNA, on the DNA of various sex factors and on the DNA of certain bacteriophages, e.g. of phage . Phage 82 DNA is sensitive only to 15-specific restriction, but not to A-specific restriction.Independently of the A- and 15-specific restrictions, the growth of phage in E. coli 15T^- encounters another limitation of yet unknown nature. No such limitation is observed either with phage 82 or with mutants of occurring at a frequency of about 10^-5.