THE STEREOCHEMICAL COURSE OF GROUP-II INTRON SELF-SPLICING

The stereochemical specificities and reaction courses for both self-sp licing steps of a group II intron have been determined by phosphorothi oate substitution at the 5' and 3' splice site phosphodiester bonds. B oth steps of the splicing reaction proceeded with a phosphorothioate i n the Sp configuration but were blocked by the Rp diastereomer. Both s teps also proceeded with inversion of stereochemical configuration aro und phosphorus, consistent with a concerted transesterification reacti on. These results are identical to those found for nuclear precursor m RNA (pre-mRNA) splicing and provide support for the hypothesis that gr oup II introns and nuclear pre-mRNA introns share a common evolutionar y history.