GROUP-I INTRONS FOUND IN CHLORELLA VIRUSES - BIOLOGICAL IMPLICATIONS

More than 80 group I introns were detected and characterized in Chlore lla viruses isolated from various locations in Japan; the overall aver age frequency of viruses containing the group I intron was 8.0%. Altho ugh most of these introns were inserted in the gene for either transcr iptional elongation factor TFIIS (similar to 60%) or URF14.2 (unidenti fied open reading frame coding for a 14.2-kDa polypeptide) (similar to 40%), in a few cases, the gene for the major capsid protein Vp52 cont ained an intron. These introns were biologically active (self-splicing ) both in vivo and in vitro. Viruses that contained introns almost usu ally contained only one, but more than two introns coexisted in severa l virus isolates. Nucleotide sequence analysis showed that the intron sequences have diverged under strong constraint of the exon genes: int rons in the same gene showed more than 99% sequence identity, whereas introns in different genes were only 72-78% identical. Phylogenetic an alysis suggested relatedness of these introns to those found in the rR NA genes of a variety of organisms including green algae, red algae, y easts, fungi, and protozoa. (C) 1998 Academic Press.