Biology of Streptomyces plasmids: Mechanism of conjugative DNA-transfer in antibiotic producing streptomycetes
As producers of antibiotics, actinomycetes are regarded as the natural source and reservoir of antibiotic resistance genes which have been developed as part of the biosynthetic gene clusters to protect the producer from its own antibiotic. By horizontal gene transfer, the resistance determinants probably found their way into pathogenic bacteria causing major health problems.
Streptomycetes contain a plethora of mobile genetic elements including small and larger circular plasmids, huge linear plasmids of several hundred kbp in size, and Actinomycete Integrative and Conjugative Elements (AICE). These elements direct the shaping of Streptomyces genomes and might have a particular role in the evolution and diversification of biosynthetic gene clusters.
With few exceptions, Streptomyces plasmids do not encode any beneficial traits for the host cell. Even small Streptomyces plasmid, like pSVH1, (http://www.ncbi.nlm.nih.gov/pubmed/16439019) having a size of 8-15 kb and replicating via the rolling-circle mechanism are self-transmissible and encode all functions required for plasmid transfer and the mobilization of chromosomal markers (Fig.1).
Streptomyces conjugation has been studied for 60 years but its underlying molecular mechanism is widely unknown. Conjugative DNA-transfer in mycelial actinomycetes is a unique process, differing considerably from conjugation via a type IV secretion system (T4SS), which has been well studied in many gram-negative and gram-positive bacteria (http://www.ncbi.nlm.nih.gov/pubmed/19946141).
