L on

■ bidirectional 1 replication from on resolve L/L'

I replicate \ resolve R'/R

Fig. 1 Replication strategies for linear replicons with hp telomeres. Three replication pathways are shown, each employing a telomere resolution step to liberate unit length linear daughters from replication intermediates. Pathway 1 shows a strategy in which the hp telomere is used as a replication origin; hp nicking at one hp telomere followed by strand displacement synthesis (several rounds of initiation and synthesis ensue) produces arrays of head-to-head, tail-to-tail, linear concatameric genomes. This strategy has been proposed for the Poxviridae (Du and Traktman 1996; Traktman 1996). Pathways 2 and 3 represent strategies employed when a bidirectional internal origin of replication is used. Pathway 2 represents the outcome of replication if both hp telomeres must be replicated before telomere resolution can ensue; an inverted repeat circular dimer intermediate would result. Pathway 3 represents the alternative, in which each replicated telomere junction is resolved as it is formed. For replicons with an asymmetrically disposed origin, Y-shaped replication intermediates would result after one rTel had been resolved. Phage N15 has been demonstrated to employ pathway 3 to maintain lysogeny (Ravin et al. 2003). It is not currently known whether B. burgdorferi uses pathway 2 or 3 or a mixture of the two pathways depending on the linear replicon

In a subsequent paper, the AT- and CG-skew analysis, which was validated for use in B. burgdorferi in the initial study, was further applied to analysis of the remaining linear replicons as well as to some of the circular plasmids (Picardeau et al. 2000). The DNA strand compositional asymmetry of the plasmids is less pronounced than that of the chromosome, but when the skew values are plotted cumulatively against locus position all the linear replicons possess a single point where the polarity of the skew switches; these points represent putative bidirectional origins. The findings suggest bidirectional origins for all the B. burgdorferi replicons, linear and circular. In other eubacterial systems typically only the chromosome utilizes bidirectional 0 -like replication, while natural plasmids use unidirectional 0 -like replication (del Solar et al. 1998).

The chromosomal origin maps to the centre of the chromosome but the CG-skew analysis suggests an asymmetrically disposed origin on some of the linear plasmids (Picardeau et al. 2000). This raises the interesting issue of when the replicated telomere junctions are processed relative to one another. If, as is the case for phage N15, one rTel is resolved before the other is even formed, then Y-shaped replication intermediates would result on the linear replicons with off-centre origins (Ravin et al. 2003; see Hertwig 2007 (this volume) for a review of the linear prophage of N15 and other closely related elements). If replication of both hp telomeres must be completed prior to resolution, then inverted repeat circular dimer intermediates would result (Fig. 1). Under conditions of moderate to fast growth bacteria uncouple replication initiation from cell division (i.e. S phase and G2/M overlap). If this also holds true for B. burgdorferi then even more complex replication intermediates would result. The modes of cellular regulation of the telomere resolution process would be predicted to be quite different for systems that utilized pathway 2 vs 3 depicted in Fig. 1.

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