Concluding Remarks and Perspectives

A large number of eukaryotic microbes and plants contain linear plasmids which are descendants of ancient viruses. Hence they afford an opportu nity to trace evolutionary mapping beginning with preeukaryotic elements to the point of vertebrate adenoviruses. In filamentous fungi such virus-like elements are exclusively confined to mitochondria; here, they commonly exhibit extremely minimalized genomes encoding basically and essentially only a DNA and an RNA polymerase. Mitochondrial linear plasmids exist either as neutral passengers within the organelle or as harmful attendants which— by integrating into the mitochondrial genome—shorten the host's life span. Studying aging in such interactions may facilitate general insights into molecular mechanisms of senescence.

The phylogenetic root of the T7-like RNA polymerase, highly conserved among mitochondrial linear plasmids, still remains obscure even though the organelles possess enzymes of that type anyway. Concerning the enzymatic repertoire needed for cytoplasmic gene expression of yeast linear plasmids, much work is to be done until a comprehensive picture of the rather unique process of cytoplasmic transcription emerges, including termination and posttranscriptional modification.

For killer toxins of either type (I and II), research will have to address early events, such as binding to the cell wall receptor and the probable transport to a yet unknown membrane receptor. Elucidating the definite mode of DNA damage occurring in type II toxin-treated cells constitutes another challenge. Investigating transmembrane passage and intracellular trafficking of the toxic subunits will not only provide basic knowledge for this rather peculiar phenomenon, but will also help to establish similarities and differences to known protein toxins of clearly diverse evolutionary origins.

Acknowledgements We thank J. Paluszynski for helpful comments on the manuscript. Financial support by the Deutsche Forschungs Gemeinschaft (DFG) grant no. ME 1142/5-1 is gratefully acknowledged.

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