Streptomyces Linear Plasmids: Their Discovery, Functions, Interactions with Other Replicons, and Evolutionary Significance

Abstract

Unusually among bacteria, streptomycetes possess linear chromosomes, and many of them also carry linear plasmids (circular plasmids are also found). The linear plasmids range in size from tens to hundreds of kilobases. The most studied is SCP1, discovered as a sex factor in the model organism Streptomyces coelicolor A3(2). A variety of co-integrates and hybrids have been found between SCP1 and the host chromosome, which can greatly increase the likelihood of chromosomal transfer. Several examples of the exchange of ends between linear plasmids and linear chromosomes have been documented. These can sometimes bring about the mobilisation of chromosomal genes for antibiotic biosynthesis. Some very large linear plasmids themselves carry genes for the biosynthesis of bioactive small molecules, including antibiotics. For example, such genes occupy about two thirds of the plasmid pSLA2-L. In another case, almost identical gene sets for methylenomycin biosynthesis are present both on the linear plasmid, SCP1, and on a quite different circular plasmid, pSV1, in a related streptomycete. It appears that linear plasmids have played key roles in the architecture, accessory gene content and rapid evolution of Streptomyces chromosomes. They may have permitted the diversification and spread of pathways for secondary metabolism, and the evolution of some Streptomyces-specific families of paralogous genes. They may also have been the source of most of the genes that, because of their possession of the rare TTA codon, are dependent for expression on the cognate tRNA specified by bldA, a gene whose deletion has wide-ranging effects on morphological differentiation and secondary metabolism.