THE MECHANISM OF U-INSERTION DELETION RNA EDITING IN KINETOPLASTID MITOCHONDRIA/

Recent advances in in vitro systems and identification of putative enz ymatic activities have led to the acceptance of a modified 'enzyme cas cade' model for U insertion/deletion RNA editing in kinetoplastid mito chondria. Models involving the transfer of uridines (Us) from the 3'-e nd of gRNA to the editing site appear to be untenable, Two types of in vitro systems have been reported: (i) a gRNA-independent U insertion activity that is dependent on the secondary structure of the mRNA; (ii ) a gRNA-dependent U insertion activity that requires addition of a gR NA that can form an anchor duplex with the pre-edited mRNA and which c ontains guiding A and G nucleotides to base pair with the added Us, In the case of the gRNA-mediated reaction, the precise site of cleavage is at the end of the gRNA-mRNA anchor duplex, as predicted by the orig inal model, The model has been modified to include the addition of mul tiple Ils to the 3'-end of the 5'-cleavage fragment, followed by the f ormation of base pairs with the guiding nucleotides and trimming back of the single-stranded oligo(U) 3'-overhang, The two fragments, which are held together by the gRNA 'splint', are then ligated, Circumstanti al in vitro evidence for involvement of an RNA ligase and an endoribon uclease, which are components of a 20S complex, was obtained, Efforts are underway in several laboratories to isolate and characterize speci fic components of the editing machinery.