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pSA1.1 is a 9.1‐kb multicopy plasmid originally isolated from Streptomyces cyaneus(formerly S. azureus) ATCC 14921. This plasmid accumulates single‐stranded DNA in S. lividansand is therefore considered to replicate by a rolling‐circle replication. In the present work, the repgene encoding the replication initiator protein and the replication origin oriof pSA1.1 were determined. The repand oriare located on separate regions. The Rep protein of pSA1.1 belongs to superfamily I which includes A proteins of phages. Nucleotide sequence of the surrounding putative nicking site of pSA1.1 shows good agreement with those of the pC194 group plasmids and phages. The direction of replication was also determined.

pSA1.1 is a 9.1-kb multicopy plasmid originally isolated from Streptomyces cyaneus (formerly S. azureus) ATCC 14921. This plasmid accumulates single-stranded DNA in S. lividans and is therefore considered to replicate by a rolling-circle replication. In the present work, the rep gene encoding the replication initiator protein and the replication origin ori of pSA1.1 were determined. The rep and ori are located on separate regions. The Rep protein of pSA1.1 belongs to superfamily I which includes A proteins of phages. Nucleotide sequence of the surrounding putative nicking site of pSA1.1 shows good agreement with those of the pC194 group plasmids and phages. The direction of replication was also determined.

Streptomyces plasmid pSA1.1 accumulated single-stranded DNA as replication intermediates in S. lividans; therefore, this plasmid was considered to replicate by a rolling-circle mechanism. A DNA-binding protein (pI > 9.7 and about 10 kDa) was purified on a denatured DNA-Cellulose column, then on a native DNA-Cellulose column. The N-terminal amino acid sequence of this protein has a high homology with bacterial histone-like proteins. In the gel retardation assay, this protein bound with the single-stranded DNA of pSA1.1. We propose that this protein may participate in the replication of pSA1.1.