Complete nucleotide sequence of the Streptomyces nigrifaciens plasmid, pSN22: genetic organization and correlation with genetic properties.

The complete nucleotide sequence of the multicopy, self-transmissible, broad-host-range Streptomyces plasmid pSN22, originating from Streptomyces nigrifaciens, was determined. pSN22 is a circular DNA molecule, 10,922 bp with 71.76% G + C. Computer-assisted analysis identified 10 open reading frames (ORFs); 8 of them--traA (155 amino acids [aa], traB (651 aa), traR (246 aa), spdB1 (107 aa), spdB2 (251 aa), spdB3 (70 aa), spdB4 (128 aa) and spdA (154 aa)--are involved in plasmid transfer and pock-formation. One ORF, rep (451 aa), probably encodes a replication protein similar to known replication proteins of rolling circle replicons. The four spdB genes have hydrophobic amino termini that might attach to the cytoplasmic membrane. The deduced rep proteins of pSN22 and pIJ101 are very similar, suggesting that both are derived from a recent common ancestor. The transfer regions of the two plasmids are, however, very different. The only detectable similarities between presumably analogous proteins are DNA- and NTP-binding motifs and hydrophobic regions. This suggests that two transfer regions are of separate origins.