Restriction endonuclease analysis of the lactose plasmid in Streptococcus lactis ML3 and two recombinant lactose plasmids

Abstract

The molecular relationship between the 60 megadalton (Mdal) recombinant lactose plasmids in ML3 .times. LM2301 lactose-positive (Lac+) transconjugants and the genetic material of S. lactis ML3 was investigated. Lactose metabolism is linked to the 33 Mdal plasmid pSK08 in ML3 and the recipient LM2301 is cured of plasmid DNA. The plasmids were analyzed with a series of restriction enzymes. The 60 Mdal plasmids in Lac+ transconjugants contained pSK08 DNA but were not simply dimers of pSK08. The 60 Mdal plasmids contained a segment of DNA not apparent in pSK08. The restriction patterns of the 60 Mdal plasmid in a Lac+ nonclumping transconjugant and that in a Lac+ clumping transconjugant were different. This suggested a molecular difference between these 2 recombinant plasmids. Evidently, the segment of DNA in the 60-Mdal plasmids that is not present in pSK08 is the transfer factor responsible for cell aggregation and high-frequency conjugation.