Retroplasmids Linear and Circular Plasmids that Replicate via Reverse Transcription

Department of Biology, Saint Louis University, St. Louis, MO 63103, USA [email protected]

1 Introduction 164

2 Circular Retroplasmids 167

2.1 Circular Retroplasmid Replication 169

3 Linear Retroplasmids 172

3.1 Linear Retroplasmid Replication 174

3.1.1 Characterization of In Vivo Replication Intermediates 174

3.1.2 Reverse Transcription 175

3.1.3 Linear Retroplasmid Replication Cycle 177

3.2 Double Hairpin Linear Retroplasmids 179

4 Evolutionary Significance 180

References 182

Abstract Retroplasmids are a unique group of genetic elements that replicate via reverse transcription. They are small, double-stranded DNAs that reside in mitochondria of certain filamentous fungal species. Retroplasmids are divided into two groups based on structure - circular and linear - and each group has distinct replication mechanisms. The reverse transcriptases (RTs) encoded by retroplasmids are deeply rooted in RT phylogeny and studies of retroplasmid replication have revealed unique mechanisms of reverse transcription that collectively suggest that retroplasmids are related to primitive genetic elements. Here, we review the types of retroplasmids that have been reported to date and focus on the studies of the replication cycle of the Mauriceville plasmid of Neurospora crassa, a prototype of circular retroplasmids, and studies of the pFOXC plasmids of Fusarium oxysporum, which are the first linear genetic elements shown to replicate via reverse transcription. The structural and mechanistic features of circular and linear retroplasmids suggest that at some point they could have been common ancestors of all known contemporary elements that replicate via reverse transcription and may hold clues to events associated with the transition of the RNA to DNA/protein world. While most mitochondrial plasmids appear to be benign, circular retroplasmids can integrate into mitochondrial DNAs and cause senescence. The significance of host-retroplasmid interactions are discussed in regard to the potential influence retroplasmids may have had on the evolution of mitochondrial DNAs and eukaryotic cells in general.

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