Detection of inteins among diverse DNA polymerase genes of uncultivated members of the Phycodnaviridae

Abstract

Viruses in the family Phycodnaviridae infect autotrophic protists in aquatic environments. Application of a PCR assay targeting the DNA polymerase of viruses in this family has revealed that phycodnaviruses are quite diverse and appear to be widespread, but a limited number of environments have been examined so far. In this study, we examined the sequence diversity among viral DNA pol genes amplified by PCR from subtropical coastal waters of O'ahu, Hawai'i. A total of 18 novel prasinovirus-like sequences were detected along with two other divergent sequences that differ at the genus-level relative to other sequences in the family. Of the 20 new sequence types reported here, three were serendipitously found to contain protein introns, or inteins. Sequence analysis of the inteins suggested that all three have self-splicing domains and are apparently capable of removing themselves from the translated polymerase protein. Two of the three also appear to be 'active', meaning they encode all the motifs necessary for a complete dodecapeptide homing endonuclease, and are therefore capable of horizontal transfer. A subsequent PCR survey of our samples with intein-specific primers suggested that intein-containing phycodnaviruses are common in this environment. A search for similar sequences in metagenomic data sets from other oceans indicated that viral inteins are also widespread, but how these genetic parasites might be influencing the ecology and evolution of phycodnaviruses remains unclear.