REQUIREMENTS FOR SELF-SPLICING OF A GROUP INTRON FROM PHYSARUM-POLYCEPHALUM

The third intron from Physarum polycephalum (Pp LSU 3) is one of the c losest known relatives to the well-studied Tetrahymena group I intron. Both introns are located at the same position in the 26S rRNA gene, a nd with the exception of an open reading frame in Pp LSU 3, are highly homologous. While Pp LSU 3 has been shown to self splice, little is k nown about its activity in vitro. We have examined the requirements fa r self splicing in greater detail. Despite its similarity to the Tetra hymena intron, Pp LSU 3 is 1500-fold less reactive, demonstrates a pre ference for high salt, and exhibits a low K-m for GTP. Removal of the open reading frame results in a modest increase of activity. This syst em provides an opportunity to understand how sequence variations in tw o related introns alter the efficiency of autoexcision, and how this r elates to adaptation of group I introns to their particular sequence c ontext.