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

Restriction and modification have been demonstrated in Streptococcus cremoris KH cells when infected by Streptococcus lactis C2 phage (designated c2) at an efficiency of plating of 2 × 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 found to be variable. From 2 to 6.5% of colonies isolated were partially deficient in restrictive capacity, permitting a greater plaquing ability by c2 phage of 1.8 to 2.9 log cycles. No completely restrictionless mutants were isolated from 1,000 colonies examined. Mutants were shown to be 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 deoxyribonucleic acid in genetically determining this specific restriction and modification system. S. cremoris KH was found to harbor 11 plasmid molecules, with molecular weights (×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.