Streptomyces Linear Plasmids: Replication and Telomeres

Abstract

Vegetative replication of terminal protein (TP)-capped linear plasmids (and linear chromosomes) of Streptomyces proceeds in two steps: a classical bidirectional replication from an internal origin followed by a novel TP-primed DNA synthesis that patches the resulting single-strand gaps at the 3′ ends (“end patching”). Replication initiation systems found on different linear Streptomyces plasmids consist of helicase-like genes and iterons of relatively diverse origins. In contrast, the end patching system (including the telomeres and the TPs) is highly conserved in most linear replicons in Streptomyces with only a single exception so far. Both the TPs and the telomeric DNAs have evolved structural features to serve replication as well as protection of the telomeres. Interaction of TPs shapes the linear replicon into a circular form, which would allow generation of biologically important superhelicity in terminal DNA, but would also create complications during postreplicational segregation of the daughter DNA. TPs may also be involved in priming initiation of replication during conjugal transfer. Like the T-DNA transfer system in Agrobacterium tumefaciens, the TPs can target themselves and the attached DNA into eukaryotic nuclei, thus suggesting a possibility of interkingdom conjugal transfer.