Mode of Transfer
In addition to cell cycle-associated replication, a linear plasmid must also be replicated during conjugal transfer to deliver a new copy to the recipient mycelium. Most linear plasmids in the Streptomyces investigated are con-jugative, being able to promote conjugation, self-transfer, and chromosome mobilization.
In typical circular plasmids of other bacteria, replication during conjugal transfer differs from the vegetative replication in initiation and mode of synthesis. Conjugal transfer is initiated by nicking of one strand of the plasmid DNA at a specific site, oriT, by a nickase (Tral in E. coli), which remains co-valently attached to the 5' end of the nicked strand. This starts a rolling circle replication around the whole plasmid, and the displaced strand is led into the recipient cell by the nickase.
Such unidirectional replication scheme, if initiated at an internal oriT of a linear plasmid, would proceed unidirectionally and replicate only part of the DNA molecule. To replicate the complete plasmid for transfer, internally initiated replication must proceed bidirectionally (as in vegetative replication). Thus, linear plasmid DNA transferred by such a mechanism would need to be in a double-stranded form to maintain continuity. There is no evidence for the double-strand transfer for linear Streptomyces plasmids, but there is experimental support for such double-strand transfer of circular plas-mids in Streptomyces (Possoz et al. 2001; Hopwood 2006; Reuther et al. 2006).
Alternatively, linear Streptomyces plasmids may be transferred from the telomeres as proposed by Chen (1996). In this "end first" model (Fig. 5), transfer of linear DNA is initiated at a telomere by a TP-primed DNA synthesis, which continues through the whole replicon and displaces the nontemplate strand from the TP-capped 5' end. As in the rolling circle model, the displaced strand is led by a protein covalently linked to the 5' end (TP) into the recipient. This conjugal replication differs from vegetative replication not only in the site of initiation, but possibly also in the substrate used by the TP-primed synthesis involved. TP-primed synthesis in the straightforward model of end patching acts on single-stranded DNA, whereas that in the proposed conjugal transfer mechanism acts on double-stranded DNA (with a TP cap), as in the case of ^29 and the adenoviruses. The latter would require a helicase for unwinding. In most linear Streptomyces plasmids, a conserved helicase gene (ttrA) is present near one or both ends. Deletion of ttrA reduces the efficiency of transfer of the plasmid, but does not affect replication of linear plasmids or chromosomes (Huang et al. 2003). ttrA homologs are also found at the ends of all Streptomyces chromosomes that have been sequenced. This,
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