Folding of the group I intron ribozyme from the 26S rRNA gene of Candida albicans

Preincubation of the group I intron Ca.LSU from Candida albicans at 37 degr eesC in the absence of divalent cations results in partial folding of this intron. This is indicated by increased resistance to T1 ribonuclease cleava ge of many G residues in most local helices, including P4-P6, as well as th e non-local helix P7, where the G binding site is located. These changes co rrelate with increased gel mobility and activation of catalysis by precurso r RNA containing this intron after preincubation. The presence of divalent cations or spermidine during preincubation results in formation of the pred icted helices, as indicated by protection of additional G residues. However , addition of these cations during preincubation of the precursor RNA alter s its gel mobility and eliminates the preincubation activation of precursor RNA seen in the absence of cations. These results suggest that, in the pre sence of divalent cations or spermidine, Ca.LSU folds into a more ordered, stable but misfolded conformation that is less able to convert into the cat alytically active form than the ribozyme preincubated without cations. Thes e results indicate that, like the group intron of Tetrahymena, multiple fol ding pathways exist for Ca.LSU. However, it appears that the role cations p lay in the multiple folding pathways leading to the catalytically active fo rm may differ between folding of these two group I introns.