Genetic evidence suggests that the gene β product of Streptococcus pyogenes plasmid pSM19035 is required for converting plasmid multimers into monomers. The β protein was purified from cells overexpressing the cloned gene. N-terminal protein sequence analysis demonstrated that the purified protein had the predicted sequence, except that the N-terminal initiator methionine was not present. Native β protein consists of a dimer of two identical subunits with a molecular mass of 23.8 kDa (25 kDa in SDS-PAGE). The β protein (isoelectric point of 9.7) binds specifically to a DNA fragment (312 bp in length) which contains the promoter region of the orf α-gene β operon and two regions (sites I and II) that show dyad axes of symmetry. It is proposed that protein β binds to sites I and II to mediate resolution of plasmid oligomers.
