Many Streptomyces species harbor circular plasmids (8 to 31 kb) as well as linear plasmids (12 to 1,700 kb). We report the characterization of two newly detected circular plasmids, pFP11 (35,139 bp) and pFP1 (39,360 bp). As on linear plasmids, their replication loci comprise repA genes and adjacent iterons, to which RepA proteins bind specifically in vitro. Plasmids containing the minimal iterons plus the repA locus of pFP11 were inherited extremely unstably; par and additional loci were required for stable inheritance. Surprisingly, plasmids containing replication loci from pFP11 or Streptomyces circular plasmid SCP2 but not from pFP1, SLP1, or pIJ101 propagated in a stable linear mode when the telomeres of a linear plasmid were attached. These results indicate bidirectional replication for pFP11 and SCP2. Both pFP11 and pFP1 contain, for plasmid transfer, a major functional traB gene (encoding a DNA translocase typical for Streptomyces plasmids) as well as, surprisingly, a putative traA gene (encoding a DNA nickase, characteristic of single-stranded DNA transfer of gram-negative plasmids), but this did not appear to be functional, at least in isolation.Streptomyces species, a major source of antibiotics and pharmacologically active metabolites, are gram-positive, mycelial prokaryotes with high GϩC content (29). They usually harbor conjugative circular and/or linear plasmids, propagating in autonomous and/or chromosomally integrating forms (17). Most known Streptomyces circular plasmids are small (8 to 14 kb) and include rolling-circle-replication (RCR) plasmids (e.g., pIJ101, pJV1, pSG5, pSN22, pSVH1, pSB24.2, and pSNA1 [11,17]) as well as chromosomally integrating/autonomous plasmids (such as SLP1 and pSAM2 [3,36,38]), whereas SCP2 is larger, at 31,317 bp (14, 45). Replication of autonomous pSAM2 occurs by an RCR mechanism (13), but the locus and mechanism of autonomous replication of SLP1 are not clear. The replication locus of non-RCR plasmid SCP2 consists of two small rep genes and adjacent noncoding sequences to which is bound one of the Rep proteins (10, 14, 32), while its mode of replication-uni-or bidirectional-has not been determined.In contrast to most eubacteria, Streptomyces species usually contain linear plasmids and linear chromosomes (15,30,34). Streptomyces linear plasmids vary in size between 12 and 1,700 kb (24, 31), with terminal inverted repeats of 0.04 to 180 kb (7, 37), and the 5Ј ends are linked covalently to terminal proteins (1, 51). Unlike the terminal-protein-capped linear replicons of adenoviruses and bacteriophage ⌽29, which replicate by a mechanism of strand displacement (43), Streptomyces linear plasmids start replication from an internally located ori locus (46) and replication proceeds bidirectionally toward the telomeres (5). At least some Streptomyces linear plasmids can also propagate in circular mode when the telomeres are deleted (5, 46). The internally located replication locus of linear plasmid pSLA2 consists of rep1 (containing iterons) and rep2 (encoding a DNA helicase...

BackgroundStreptomyces coelicolor is the most studied Streptomyces species and an excellent model for studying differentiation and antibiotic production. To date, many genes have been identified to be required for its differentiation (e.g. bld genes for aerial growth and whi genes for sporulation) and antibiotics production (including actII-orf4, redD, cdaR as pathway-specific regulatory genes and afsR, absA1/A2 as pleiotropic regulatory genes).ResultsA gene cluster containing six genes (SCO4126-4131) was proved to be co-transcribed in S. coelicolor. Deletions of cmdABCDEF (SCO4126-4131) displayed defective sporulation including formation of aberrant branches, and abnormalities in chromosome segregation and spore septation. Disruption mutants of apparently orthologous genes of S. lividans and S. avermitilis also showed defective sporulation, implying that the role of these genes is similar among Streptomyces. Transcription of cmdB, and therefore presumably of the whole operon, was regulated developmentally. Five of the encoded proteins (CmdA, C, D, E, F) were predicted membrane proteins. The other, CmdB, a predicted ATP/GTP-binding protein with an ABC-transporter-ATPase domain shown here to be essential for its function, was also located on the cell membrane. These results indicate that CmdABCDEF proteins mainly affect Streptomyces differentiation at an early stage of aerial hyphae formation, and suggest that these proteins may form a complex on cell membrane for proper segregation of chromosomes. In addition, deletions of cmdABCDEF also revealed over-production of blue-pigmented actinorhodin (Act) via activation of transcription of the pathway-specific regulatory gene actII-orf4 of actinorhodin biosynthesis.ConclusionIn this study, six co-transcribed genes cmdABCDEF were identified by their effects on differentiation and antibiotic production in Streptomyces coelicolor A3(2). These six membrane-located proteins are possibly assembled into a complex to function.

Here we report that tgdA, a novel gene encoding a putative transglycosylase, affects both the morphological differentiation and the yield of blue-pigmented compound actinorhodin in Streptomyces coelicolor. The tgdA null mutant displays sparse aerial hyphae and irregular spore chains frequently lacking chromosomal DNA. Elevated actinorhodin production coincides with the overexpression of actII-orf4 in mutant. tgdA expression is temporally and developmentally regulated. The tgdA orthologs in Streptomyces avermilitis and Streptomyces lividans also affect differentiation.