A 6.5-kb DNA fragment containing the gene (rpoB) encoding the RNA polymerase (RNAP) β subunit, from the mollicute Spiroplasma citri (Sc), was cloned and sequenced. The classical eubacterial organization, with the genes (rplK, A, J and L) encoding ribosomal proteins L11, L1, L10 and L12 located immediately upstream from rpoB, was not found in the Sc DNA. Instead, an open reading frame (hsdS) potentially encoding a component of a type I restriction and modification system was identified upstream from rpoB, and sequences showing similarities with insertion elements were found between hsdS and rpoB.