The naturally temperature-sensitive plasmid pSG5 of Streptomyces ghanaensis DSM 2932 is the basis replicon of the "pGM-vectors." The nucleotide sequence of the pSG5 minimal replicon was determined. Only one single open reading frame (rep) with high coding probability is located on the minimal replicon. The deduced Rep protein consists of 378 aa and contains motifs characteristic of initiation proteins for rolling-circle-type replication. Sequence similarity indicated that the Rep protein of pSG5 is related to the Rep proteins of the pC194 plasmid family. Accumulation of large amounts of single-stranded plasmid DNA was shown for all small pGM vectors. A minus origin for the lagging strand synthesis was localized outside of the pSG5 minimal replicon. Although the sequenced pSG5 fragment is self-transmissible, it seems not to carry further genes in addition to the rep gene. This suggests that the transfer mechanism of plasmid pSG5 differs from that of other Streptomyces plasmids, which all encode specific transfer genes.