The mode of action of how protelomerases generate the covalently closed ends of the three known linear plasmid prophages has been elucidated to a large extent. Although most of the work has been done on the N15 pro-telomerase TelN, the homologies to TelK and TelY and the similar target sites of the protelomerases suggest the same principle by which the processing of the telomere resolution site occurs. To define the domains of the protelom-erases involved in target binding, cleavage and joining, structural analyses of the proteins certainly would supply valuable information. Another aspect to be clarified pertains to the regulation of protelomerase synthesis. This enzyme is essential for the establishment and maintenance of the lysogenic cycle. However, it can be predicted that protelomerase activity would perturb lytic replication by cleaving the linear phage genomes. Hence the question arises of how protelomerase activity ceases upon induction. Transcription studies disclosed expression of the protelomerase genes of N15 and PY54 at the late stage of lytic growth (Hertwig et al. 2003b; Ravin et al. 2000). Huang et al. (2004) reported that TelK may act stoichiometrically and becomes inactive at the end of one transesterification. This scenario would provide a measure to ensure that no excess activity is present when it is no longer needed. It is also conceivable that protelomerase activity may be abolished by inactivation of the protein, by degradation or by an inhibitor protein that has not yet been identified. However, it cannot be excluded that the pro-telomerase also plays a role during lytic replication.

Acknowledgements The author thanks Erich Lanka for constructive discussions. The assistance of Jens A. Hammerl in preparing the figures is highly appreciated.

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