Complete sequence and analysis of the stability functions of pPSX, a vector that allows stable cloning and expression of Streptomycete genes in Escherichia coli K12.

The broad host range, cloning and expression vector pPSX has been completely sequenced and analysed. pPSX is 14.7kb in length and contains the fusion of two continuous segments of the parental 34kb, IncW plasmid pR388. pPSX appears to have retained at least three sets of gene/s which contribute in different ways to plasmid stability. The first of these parB, is a known participant in the partitioning of low-copy number plasmids. While the adjoining gene, orf35, has high homology with kfrA, a putative plasmid nucleoid organiser that is often associated with the ParAB family of proteins. The second set of genes; orfs18, 19, 20, whose exact functions are not clear, have homology to the stability operons of both IncW and IncN plasmids. The third is the resolvase, resP, which may resolve plasmid multimers that can lead to plasmid instability. pPSX is a small, stable cloning vector good for cloning and expression of a wide range of genes, including those from streptomycetes.