Sequence relationships between plasmids associated with conventional MLS resistance and zonal lincomycin resistance...

By using electron microscopy of self-annealed DNA and restriction enzyme analysis, we have compared the physical maps of two group A streptococcal plasmids associated with conventional MLS resistance (pEL1; 20 Md) and zonal lincomycin resistance (pSM10419; 15 Md). Of their monomeric molecules, about 40% and 60%, respectively, are occupied by identical non-tandem inverted repeats containing sequences specifying putative replication functions. Sequence homology also exists between their resistance determinants which are located in unique DNA. Moreover, homology between additional regions of unknown function is so extensive and restriction fragment arrangement so similar that, formally, pSM10419 can be considered a deletion variant of pEL1. The results suggest that MLS and zonal lincomycin resistance have the same biochemical basis (i.e. methylation of 23S ribosomal RNA) and differ only quantitatively in the inducible control systems.