Linear plasmid SLP2 of Streptomyces lividans is a composite replicon

SLP2 is a 50 kb linear plasmid in Streptomyces lividans that contains short (44 bp) terminal inverted repeats and covalently bound terminal proteins. The nucleotide sequence of SLP2 was determined. The rightmost 15.4 kb sequence is identical to that of the host chromosome, including the Tn4811 sequence at the border, which is interrupted by an insertion sequence (IS) element in SLP2. Examination of the flanking target sequences of Tn4811 suggests a previous recombinational event there. The 43 putative protein coding sequences contained many involved in replication (including two terminal protein homologues), partitioning, conjugal transfer and intramycelial spread. The terminally located helicase-like gene ttrA was necessary for conjugal transfer. The two telomeres diverge significantly in primary sequence, while preserving similar secondary structures. Mini-linear plasmids containing these telomeres replicated in S. lividans using the chromosomally encoded terminal protein. In addition, two pseudotelomere sequences are present near the left telomere. The G+C content and GC or AT skew profiles exhibit complex distributions. These, plus the inferred recombination at the right arm, indicate that SLP2 has evolved through rounds of exchanges involving at least three replicons.