Abstract
Streptomyces linear plasmids and linear chromosomes can replicate also in a circular form when their telomeres are deleted. The 17-kb linear plasmid pSLA2 has been a useful model in studies of such replicons. Here we report that the minimal origin initiating replication of pSLA2-derived plasmids as circular molecules cannot propagate these plasmids in a linear mode unless they also contain a novel plasmid-encoded locus, here named rlrA (required for linear replication). In contrast with the need for rlrA to accomplish replication of telomere-containing linear plasmids, expression of rlrA, which encodes two LuxR family regulatory domains, interferes with the establishment of pSLA2 in circular form in Streptomyces lividans transformants. The additional presence of an adjacent divergently transcribed locus, rorA (rlrA override), which strongly resembles the kor (kil override) transcription control genes identified previously on Streptomyces plasmids, reversed the detrimental effects of rlrA on plasmid establishment and additionally stabilized circular plasmid inheritance by spores during the S. lividans life cycle. While the effects of the rlrA/rorA locus of pSLA2 were seen also on linear plasmids derived from the unrelated SLP2 replicon, they did not extend to plasmids whose replication was initiated at a cloned chromosomal origin. Our results establish the existence of, and provide the initial description of, a novel plasmid-borne regulatory system that differentially affects the propagation of linear and circular plasmids in Streptomyces.