Sequence relationships between plasmids carrying genes for lactose utilization

Southern hybridization experiments carried out between pSC101::Tn951 DNA and lambda dlac DNA allowed the location and orientation of the lac operon within the transposon to be deduced. The same method was used to detect Tn951 on Lac plasmids from 11 independent isolates from three continents. None of these plasmids was found to carry an entire Tn951 sequence but they all contained lac genes homologous to the lac genes of Tn951. The lac operon of one of these plasmids was bordered by a sequence homologous to that found at the left-hand side of Tn951. It is concluded that the lac determinants of the Lac plasmids analysed and of Tn951 have evolved from a common ancestor but that the distribution of these determinants cannot be attributed to a spread of the transposon Tn951.