Retroplasmids: Linear and Circular Plasmids that Replicate via Reverse Transcription

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 eachgroup has distinct replication mechanisms. The reverse transcriptases (RTs) encoded by retroplasmids aredeeply rooted in RT phylogeny and studies of retroplasmid replication have revealed unique mechanisms ofreverse 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 thereplication cycle of the Mauriceville plasmid of Neurospora crassa, a prototypeof 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 structuraland mechanistic features of circular and linear retroplasmids suggest that at some point they could havebeen common ancestors of all known contemporary elements that replicate via reverse transcription and mayhold clues to events associated with the transition of the RNA to DNA/protein world. While most mitochondrialplasmids 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 influenceretroplasmids may have had on the evolution of mitochondrial DNAs and eukaryotic cells in general.