SummaryLinear plasmids and chromosomes of Streptomyces carry terminal proteins (TPs) covalently attached to the 5Ј ends of the DNA. Most known telomeres are conserved in primary sequence and in the potential secondary structures formed during replication. The TP that caps these telomeres is also highly conserved and its coding gene, tpg, is present in all Streptomyces chromosomes and some linear plasmids. Linear plasmid SCP1 contains atypical telomere sequences and no tpg homologue, and can replicate in the absence of tpg, suggesting that it carries a novel TP gene. To isolate the TP on the SCP1 telomeres, we constructed a multicopy mini-SCP1 plasmid. The TP capping the plasmid was isolated and subjected to tryptic digestion and mass spectrometric analysis, and the results indicated that the TP was encoded by an open reading frame (ORF), SCP1.127 (tpc), on SCP1. Of the two ORFs upstream of tpc, SCP1.125 (tac) but not SCP1.126 was essential for replication of mini-SCP1. The Tac-Tpc system of SCP1 represents a convergently evolved novel telomere-capping system of Streptomyces linear replicons.

Linear chromosomes and linear plasmids of Streptomyces possess covalently bound terminal proteins (TPs) at the 5′ ends of their telomeres. These TPs are proposed to act as primers for DNA synthesis that patches the single-stranded gaps at the 3′ ends during replication. Most (‘archetypal’) Streptomyces TPs (designated Tpg) are highly conserved in size and sequence. In addition, there are a number of atypical TPs with heterologous sequences and sizes, one of which is Tpc that caps SCP1 plasmid of Streptomyces coelicolor. Interactions between the TPs on the linear Streptomyces replicons have been suggested by electrophoretic behaviors of TP-capped DNA and circular genetic maps of Streptomyces chromosomes. Using chemical cross-linking, we demonstrated intramolecular and intermolecular interactions in vivo between Tpgs, between Tpcs and between Tpg and Tpc. Interactions between the chromosomal and plasmid telomeres were also detected in vivo. The intramolecular telomere interactions produced negative superhelicity in the linear DNA, which was relaxed by topoisomerase I. Such intramolecular association between the TPs poses a post-replicational complication in the formation of a pseudo-dimeric structure that requires resolution by exchanging TPs or DNA.