The large linear plasmid pSLA2-L of Streptomyces rochei has an unusually condensed gene organization for secondary metabolism.

The complete nucleotide sequence of the large linear plasmid pSLA2-L in Streptomyces rochei strain 7434AN4 has been determined. pSLA2-L was found to be 210 614 bp long with a GC content of 72.8% and carries 143 open reading frames. It is especially noteworthy that three-quarters of the pSLA2-L DNA is occupied by secondary metabolism-related genes, namely two type I polyketide synthase (PKS) gene clusters for lankacidin and lankamycin, a mithramycin synthase-like type II PKS gene cluster, a carotenoid biosynthetic gene cluster and many regulatory genes. In particular, the lankacidin PKS is unique, because it may be a mixture of modular- and iterative-type PKSs and carries a fusion protein of non-ribosomal peptide synthetase and PKS. It is also interesting that all the homologues of the afsA, arpA, adpA and strR genes in the A-factor regulatory cascade in Streptomyces griseus were found on pSLA2-L, and disruption of the afsA homologue caused non-production of both lankacidin and lankamycin. These results, together with the finding of three possible replication origins at 50-63 kb from the right end, suggest that the present form of pSLA2-L might have been generated by a series of insertions of the biosynthetic gene clusters into the left side of the original plasmid.