Plasmids in Streptococcus lactis: evidence that lactose metabolism and proteinase activity are plasmid linked

Abstract

Populations of lactose positive (Lac+) and proteinase (Prt+) cells from S lactis M18, C10 and ML3 grown at 39°C gave rise to increasing proportions of Lac-Prt- clones. The deficiencies did not appear until after a number of generations at the elevated temperature, and the rate depended on the strain. Lac-Prt+ and Lac+Prt- mutants were isolated after treatment with ethidium bromide. Plasmid deoxyribonucleic acid was isolated by cesium chloride ethidium bromide equilibrium density gradient centrifugation from the parent cultures as well as from their Lac-Prt-, Lac-Prt+, and Lac+Prt- mutants. Five distinct plasmid sizes of approximate molecular weights of 2, 4, 8, 21, and 27 million were found in S lactis C10, whereas the Lac-Prt- derivative lacked the 8 and 21 million dalton plasmids, but the 8 million dalton plasmid was present in the Lac-Prt+ mutant. In S lactis M18 five plasmids possessing molecular weights about 2, 4, 10, 18, and 27 million were observed. The 10 and 18 million dalton plasmids were not detected in the Lac-Prt- mutants, whereas the Lac-Prt+ derivative lacked only the 18 million dalton plasmid and the Lac+Prt- mutant lacked only the 10 million dalton plasmid. In S lactis ML3 five distinct plasmids, with approximate molecular weights of 2, 4, 8, 22, and 30 million were present The 8 and 22 million dalton plasmids were not detected in the Lac-Prt- derivative, but the 8 million dalton plasmid was present in the Lac-Prt+ mutant. The evidence suggests that lactose fermenting ability and proteinase activity in these organisms are mediated through two distinct plasmids having molecular weights of 8 x 106 to 10 x 106 for proteinase activity and 18 x 106 to 22 x 106 for lactose metabolism.