JB Accepts, published online ahead of print on 30 October 2009

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J. Bacteriol. doi:10.1128/JB.01192-09
Copyright (c) 2009, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Linear plasmid SLP2 is maintained by partitioning, intrahyphal spread, and conjugal transfer in Streptomyces

Chin-Chen Hsu and Carton W. Chen^*

Department of Life Sciences and Institute of Genome Sciences, National Yang-Ming University, Shih-Pai, Taipei 112, Taiwan

^* To whom correspondence should be addressed. Email: cwchen{at}ym.edu.tw.

Low-copy-number plasmids generally encode a partitioning system to ensure proper segregation after replication. Little is known about partitioning of linear plasmids in Streptomyces. SLP2 is a 50-kb low-copy-number linear plasmid in Streptomyces lividans, which contains a typical parAB partitioning operon. In S. lividans and S. coelicolor, a parAB deletion resulted in moderate plasmid loss and growth retardation of colonies. The latter was caused by conjugal transfer from plasmid-containing to plasmid-less hyphae. Deletion of the transfer (traB) gene eliminated conjugal transfer, lessened the growth retardation of colonies, and increased plasmid loss through sporulation cycles. Additional deletion of an intrahyphal spread gene (spd1) caused almost complete plasmid loss in a sporulation cycle, and eliminated all growth retardation. Moreover, deletion of spd1 alone severely reduced conjugal transfer and stability of SLP2 in S. coelicolor M145, but had no effect in S. lividans TK64. These results revealed three systems for SLP2 maintenance: partitioning and spread for moving the plasmid DNA along the hyphae and into spores, and conjugal transfer for rescuing plasmid-less hyphae. In S. lividans, both spread and partitioning appear to overlap functionally, but, in S. coelicolor, spread appears to play the main role.