REPLACEMENT OF 2 NON-ADJACENT AMINO-ACIDS IN THE SACCHAROMYCES-CEREVISIAE BI2 INTRON-ENCODED RNA MATURASE IS SUFFICIENT TO GAIN A HOMING-ENDONUCLEASE ACTIVITY

Two homologous group I introns, the second intron of the cyt b gene, f rom related Saccharomyces species differ in their mobility, The S.cape nsis intron is mobile and encodes the I-ScaI endonuclease promoting in tron homing, whilst the homologous S.cerevisiae intron is not mobile, but functions as an RNA maturase promoting splicing. These two intron- encoded proteins differ by only four amino acid substitutions, Taking advantage of the remarkable similarity of the two intron open reading frames and using biolistic transformation of mitochondria, we show tha t the replacement of only two non-adjacent residues in the S.cerevisia e maturase carboxy-terminal sequence is sufficient to induce a homing- endonuclease activity without losing the splicing function, Also, we d emonstrate that these two activities reside in the S.capensis bi2-enco ded protein which functions in both splicing and intron mobility in th e wild-type cells, These results provide new insight into our understa nding of the activity and the evolution of group I intron-encoded prot eins.