Identification and Cloning of Plasmid Deoxyribonucleic Acid Coding for Abortive Phage Infection from Streptococcus lactis ssp. diacetylactis KR2

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Abstract
The phage insensitivity mechanism of Streptococcus lactis ssp. diacetylactis KR2 was investigated. This strain harbored seven detectable plasmids ranging in size from 2.9 to 32 Mdaltons. Transformation of KR2 plasmid DNA into plasmid-free Streptococcus lactis LM0230 yielded lactose-positive transformants that were either sensitive or exhibited a reduced sensitivity to c2 phage. The latter possessed plasmids of 23.6 and 29 Mdaltons, designated pKR223 and pKR229, respectively. Lactose-positive phage-sensitive transformants contained only pKR229. Transformants exhibiting reduced phage sensitivity were cured of lactose-fermenting ability. Lactose-negative derivatives retained reduced phage sensitivity and possessed only pKR223, thus linking this phenotype to the 23.6-Mdalton plasmid of KR2. The gene or genes responsible for the reduced phage sensitivity phenotype were cloned on a 17-kb HpaII fragment of pKR223 into the single HPaII site on the streptococcal cloning vector, pGB301. The phage defense mechanism(s) encoded by pKR223 and the 17-kb fragment of pKR223 involved a reduction in plaque size, which appeared to be due to reduced phage burst size and abortive phage infection. The data provided evidence that the phage insensitivity phenotype exhibited by Strep. lactis ssp. diacetylactis KR2 is, in part, linked to a 23.6-Mdalton plasmid coding for a reduced sensitivity to prolate phage.