Modified base-binding EVE and DCD Domains Implicated in the Origins of Programmed Cell Death and the piRNA Pathway

BackgroundDNA and RNA of most cellular life forms and many viruses contain an expansive repertoire of modified bases. The modified bases play diverse biological roles that include both regulation of transcription and translation, and protection against restriction endonucleases and antibiotics. Modified bases are often recognized by dedicated protein domains. However, the elaborate networks of interactions and processes mediated by modified bases are far from being completely understood. ResultsWe present a comprehensive census and classification of EVE domains that belong to the PUA/ASCH domain superfamily and bind various modified bases in DNA and RNA. Prokaryotes encode two classes of EVE domain proteins, slow-evolving and fast-evolving. The slow-evolving EVE domains in -proteobacteria are embedded in a conserved operonic context that implies involvement in coupling between translation and respiration, in particular, cytochrome c biogenesis, potentially, via binding 5-methylcytosine in tRNAs. In {beta} and {gamma}-proteobacteria, the conserved associations implicate the EVE domains in the coordination of cell division, biofilm formation, and global transcriptional regulation by non-coding 6S small RNAs, which are potentially modified and bound by the EVE domains. Down-regulation of the EVE-encoding operons might cause dormancy or programmed cell death (PCD). In eukaryotes, the EVE-domain-containing THYN1-like proteins appear to inhibit PCD and regulate the cell cycle, likely, via binding 5-methylcytosine and its derivatives in DNA and/or RNA. Thus, the link between PCD and cytochrome c that appears to be universal in eukaryotes might have been inherited from the -proteobacterial, proto-mitochondrial endosymbiont and, unexpectedly, could involve modified base recognition by EVE domains. In numerous prokaryotic genomes, fast-evolving EVE domains are embedded in defense contexts, including toxin-antitoxin modules and Type IV restriction systems, all of which can also induce PCD. These EVE domains likely recognize modified bases in invading DNA molecules and target them for restriction. We additionally identified EVE-like prokaryotic Development and Cell Death (DCD) domains that are also implicated in defense functions including PCD. This function was inherited by eukaryotes but, in animals, the DCD proteins apparently were displaced by the extended Tudor family, whose partnership with Piwi-related Argonautes became the centerpiece of the piRNA system. ConclusionsRecognition of modified bases in DNA and RNA by EVE-like domains appears to be an important, but until now, under-appreciated, common denominator in a variety of processes including PCD, cell cycle control, antivirus immunity, stress response and germline development in animals.