Group I introns found in Chlorella viruses: Biological implications

Abstract

More than 80 group I introns were detected and characterized in Chlorella viruses isolated from various locations in Japan; the overall average frequency of viruses containing the group I intron was 8.0%. Although most of these introns were inserted in the gene for either transcriptional elongation factor TFIIS (~60%) or URF14.2 (unidentified open reading frame coding for a 14.2-kDa polypeptide) (~40%), in a few cases, the gene for the major capsid protein Vp52 contained an intron. These introns were biologically active (self-splicing) both in vivo and in vitro. Viruses that contained introns almost usually contained only one, but more than two introns coexisted in several virus isolates. Nucleotide sequence analysis showed that the intron sequences have diverged under strong constraint of the exon genes: introns in the same gene showed more than 99% sequence identity, whereas introns in different genes were only 72-78% identical. Phylogenetic analysis suggested relatedness of these introns to those found in the rRNA genes of a variety of organisms including green algae, red algae, yeasts, fungi, and protozoa.