Evidence for Plasmid Linkage of Restriction and Modification in Streptococcus cremoris KH

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Abstract
Restriction and modification were demonstrated in S. cremoris KH cells when infected by S. lactis C2 phage (designated c2) at an efficiency of plating of 2 .times. 10-7. The growth of c2 phage through KH cells produces modified progeny phage capable of unrestricted growth on KH cells. The ability of single-colony isolates of S. cremoris KH cultures to restrict and modify c2 phage was variable. From 2-6.5% of colonies isolated were partially deficient in restrictive capacity, permitting a greater plaquing ability by c2 phage of 1.8-2.9 log cycles. No completely restrictionless mutants were isolated from 1000 colonies examined. Mutants were deficient in both restriction and modification capabilities of the same specificity. The frequent occurrence of a genotypic change that resulted in the loss of both restriction and modification capacities indicated the involvement of plasmid DNA in genetically determining this specific restriction and modification system. S. cremoris KH harbored 11 plasmid molecules, with MW (.times. 106) estimated to be 50, 41, 24, 18, 10, 7.4, 3.3, 3.0, 2.8, 2.5 and 1.5. Of the 27 mutants examined, 25 were missing the 10-megadalton plasmid. This consistent plasmid difference among the majority of mutants isolated supports the involvement of this plasmid in restriction and modification. Plasmid linkage of restriction and modification systems provides a genetic mechanism for the rapid development of phage-sensitive starter cultures due to the inherent instability of extrachromosomal elements.