The Genome Sequence of the Phycodnavirus, EhV-8

The family Phycodnaviridae, contain viruses that infect both freshwater and marine algae.  These viruses, known as nucleocytoplasmic large DNA viruses (NCLDV), complete their replication in the cytoplasm of a host cell particular to that species.  There are two other Phycodnaviridae that have been sequenced in recent years.  EsV-1possessing 33,593 base pairs (bp), known to infect the marine algae Ectocarpus siliculosus, and PBCV-1 that contains 330,744-bp and infects a unicellular algal symbiont, Paramecium bursaria.

            The virus sequenced in this study, EhV-86, is known to infect the coccolithophore, Emiliania huxleyi; known for it’s stupendous milky blooms found in temperate regions and a key species in climate modeling and international biogeochemical cycles.

Sequencing of the EhV-86 virus revealed it to be a new genus of the Phycodnaviridae family, aptly named Coccolithovirus.  The analysis reveled a number of surprising finds; the genome of EhV-86 has 407,339 bp is the largest of any Phycodnavirus; only 14% of the coding sequences (CDSs) found were matched with functional products from a systematic gene finder and most of the genes with functionality were at the ends of the genome rendering the central areas indistinct. However, using Microarray analysis, it was found that these central CDSs were expressed 33hrs into the infection of E. Huxleyi culture, alluding to a functionality.

            The EhV-86 homologs showed several genes that had previously never been found in viruses.  These were mostly used in sphingolipid biosynthesis, which are present in all eukaryotes and some prokaryotes. This had also never been documented in viruses before.  It is hypothesized that EhV-86 prompts apoptosis in it’s host cell and broadcast spawns virons as part of it’s infection cycle.  Formerly, apoptosis in algal cells had only been documented as a response to physiological stresses such as temperature.

            EhV-86 also has six unique RNA polymerase sub-units not found in other Phycodnaviruses.  The presence of these expressed sub-units suggests that EhV-86 encodes it’s own transcription gear. As other Phycodnaviruses do not have these RNA polymerase sub-units, further analysis may reveal it to be a separate sub-family.

The characteristics of EhV-86 reveals unique characteristics and functions previously only found in eukaryotes and prokaryotes and implies a new foundation of lipids in the marine food chain. 

Wilson et al, 2005. Complete genome sequence and lytic phase transcription profile of a coccolithovirus. Science. 309: 1090-1092.