Electron microscopic mapping of deletions on a streptoccocal plasmid carrying extraordinarily long inverted repeats

Deletions AlOl, A102, and Al03 which occurred within the extraordinarily long inverted repeats of the self-ligated large EcoRI fragment of the streptococcal MLS (macrolides, lincosamides, streptogramin B)-resistance plasmid pSMl9035 led to the formation of plasmids pDBlO1, pDBl02, and pDB103. Their molecular lengths were determined by contour length measurements to be 17.8, 17.4, and 13.9 kb, respectively. Electron micro-scopic examination of self-annealed molecules revealed stem-loop structures with inverted repeats comprising 41 to 91 % of the mass of plasmids. Two unique sequences (US1 and US,) separated the inverted repeats in the case of pDBlO1 and pDBl03, while in pDB102 the repeats were joined at one end and separated at the other by a unique sequence (US,). The size of the unique sequence US, was identical for all three plasmids, and the location of the resistance determinant was determined by electron microscopic examination of self-annealed molecules of the recombinant plasmid pDB201. Mapping of the deletion termini, accomplished by combining electron microscopic and Hind111 restriction data, suggested that deletions may occur at preferential sites. The l&MdaP streptococcal plasmid pSM