Fig. 5 "End first" model of conjugal transfer of linear plasmids and chromosomes of Streptomyces compared to the rolling circle model of conjugal transfer of circular repli-cons. Left: In E. coli, F plasmid-specific proteins carry out a nicking-priming event at oriT on F DNA, where the 5' end is covalently attached to the nickase (Tral), and a rolling circle replication is initiated at the 3' end. The Tral-capped 5' strand is continuously displaced and transferred to the recipient. In Hfr cells, the chromosomal DNA is led by the F sequence into the recipient cell as an exogenote. Right: For linear plasmids and linear chromosomes of Streptomyces, the TP-capped telomere may act as the origin of transfer (oriT), where TP-primed replication is initiated, and the TP-capped 5' strand is displaced and transferred, as for F/Hfr transfer so far, is the only experimental support for the end-first model of transfer. As in the case of vegetative replication, it is not clear which DNA polymerase(s) is involved in replication during conjugal transfer.
Both Tpgs and Tpc contain a nuclear localization signal (NLS) or NLS-like sequence. A monopartite NLS was initially predicted in the TP of S. coelicolor (TpgSco) and S. lividans (TpgSh) (Yang et al. 2002). Similar motifs have been found at the same locations in many other Tpgs (Fig. 2a). A bipartite NLS is also found on Tpc (Fig. 2b). The finding of two different types of NLSs in two con-vergently evolved TP families raised the likelihood that their occurrences are not fortuitous, although Streptomyces, being prokaryotic, lacks nuclei. A recent study (H.-H. Tsai et al., unpublished results) demonstrated that these NLSs can function in targeting the TPs (TpgSco, TpgSav, and Tpc) and the attached DNA into the nuclei of human cells (Fig. 6). The NLS alone is sufficient to perform nuclear targeting. On the other hand, the NLSs are dispensable for replication of the linear plasmids, suggesting that the nuclear localization function in these TPs has evolved independently of their replication function.
The ability of the TPs to carry the attached DNA into eukaryotic nuclei is analogous to the interkingdom transfer of T-DNA from Agrobacterium tume-faciens to plant nuclei by its capping protein (VirD), which also possess NLSs
Fig. 6 Nuclear targeting function of TPs. tpgSco was translationally fused to the N-terminus of a reporter gene, EGFP3 (encoding a triple green fluorescence protein concatamer), under the control of the CMV immediate early promoter (Pcmv) on pEGFP3 vector (top), and used to transfect human HeLa cells. In the transfected cells, green fluorescence accumulated in the nuclei (bottom). On the left is a phase-contrast microscopic image, and on the right is a fluorescence microscopic image of the same transformed cells. In the cells transfected by pEGFP3 vector, the fluorescence was present mainly in the cytosol
(for review, see Zambryski et al. 1989). This raises the possibility that TP-mediated DNA transfer may also take place in nature, between Streptomyces and, for example, plants in soil. This possibility, which has great evolutionary and ecological significance, remains to be investigated.
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