Concluding Remarks

The Streptomyces TPs share no homology with those of ^29 and adenoviruses, indicating a convergent evolution of TP-primed synthesis across various systems. Their small size (184-185 aa for Tpgs and 259 aa for Tpc) are amazing, considering the diversity of possible functions they may possess including: (1) priming DNA synthesis during end patching (and perhaps replication during conjugal transfer); (2) protecting against exonucleolytic attacks; (3) providing topological constraints through TP-TP interactions to achieve superhelicity; and (4) nuclear targeting in eukaryotic cells.

As expected from their frequent exchanges, the linear plasmids of Streptomyces have the same telomere structures as their host chromosomes. The plasmid telomere sequences exhibit wider divergence (Huang et al. 1998), probably reflecting their high mobility and a larger latitude for experimentation. Initiation of replication of the linear chromosomes of Streptomyces is similar to that of circular bacterial chromosomes in terms of participating enzymes and oriC organization (Jakimowicz et al. 1998). Similarly, studies on the initiation of replication of the linear plasmids, although limited, have so far revealed no surprises. The major interest in replication of these linear replicons has thus been focused on the novel end patching process, and some progress has been made in this area. Although the studies are applicable to both linear plasmids and chromosomes, the plasmids are much easier to manipulate in the laboratory.

So far, most end patching studies have focused on the archetypal telom-eres and TPs. The appearance of novel telomeres and capping TP systems in linear plasmids (and probably chromosomes) promises to provide interesting variations on the same theme with their own clues. Beyond end patching, there lie a number of more complicated issues. Due to the presence of the telomere-telomere interaction, these linear replicons may face the same postreplicational topological issues as circular DNA molecules, such as resolution of the concatemers (by Topo IV) and pseudodimers (by recombination or TP swapping; Fig. 4).

In addition to vegetative replication, another type of replication takes place during conjugal transfer. Such replication is typically initiated at a specialized origin (oriT) on the plasmid (free or integrated), different from that (oriP) for vegetative replication. Although the end first model of transfer is attractive, it has not been confirmed. Finally, if the nuclear targeting of TPs in eukary-otic cells is part of a natural exchange system with certain eukaryotes, such an interkingdom transfer system begs for rigorous investigation.

Acknowledgements I thank Ralph Kirby, Hung-Wei Hsu, and Ting-Wen Chen for bioin-formatic assistance, Zhongjun Qin and Haruyasu Kinashi for communications of unpublished results, and David Hopwood, Keith Chater, Haruyasu Kinashi, Ralph Kirby, and Hsiu-Hui Tsai for critical reading of the manuscript and suggestions for improvements. This study is supported by grant from the National Science Council, R. O. C. (NSC95-2321-B010-005,) and a grant from the Ministry of Education, R. O. C. (Aim for the Top University Plan).

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