Chaperonins are protein-folding machinery found in all cellular life. Chaperonin 
genes have been documented within a few viruses, yet, surprisingly, analysis of 
metagenome sequence data indicated that chaperonin-carrying viruses are common 
and geographically widespread in marine ecosystems. Also unexpected was the 
discovery of viral chaperonin sequences related to thermosome proteins of 
archaea, indicating the presence of virioplankton populations infecting marine 
archaeal hosts. Virioplankton large subunit chaperonin sequences (GroELs) were 
divergent from bacterial sequences, indicating that viruses have carried this 
gene over long evolutionary time. Analysis of viral metagenome contigs indicated 
that: the order of large and small subunit genes was linked to the phylogeny of 
GroEL; both lytic and temperate phages may carry group I chaperonin genes; and 
viruses carrying a GroEL gene likely have large double-stranded DNA (dsDNA) 
genomes (>70 kb). Given these connections, it is likely that chaperonins are 
critical to the biology and ecology of virioplankton populations that carry 
these genes. Moreover, these discoveries raise the intriguing possibility that 
viral chaperonins may more broadly alter the structure and function of viral and 
cellular proteins in infected host cells.