Comparative analysis of splicing of the complete set of chloroplast group II introns in three higher plant mutants

The barley mutant albostrians and the maize mutants crs1 and crs2 are defec tive in the splicing of various plastid group II introns, By analysing tRNA precursors and several mRNAs not previously examined, the investigation of in vivo splicing defects in these mutants has been completed. The albostri ans mutation causes the loss of plastid ribosomes resulting secondarily in a disruption of splicing of all subgroup IIA introns in the chloroplast, Th us MatK, the only putative chloroplast intron-specific maturase of higher p lants, might have evolved to function in splicing of multiple introns, We s how that in the case of tRNA-Ala(UGC) the first step of splicing is affecte d, as suggested by the absence of lariat molecules, Thus the plastid-encode d splicing factor lacking in albostrians must participate in the formation of the catalytically active structure. In contrast, a mutation in the nucle ar gene crs1 prevents splicing of only one intron but causes specific addit ional effects as precursor transcripts for tRNA-lle(GAU), tRNA-Ala(UGC), tR NA-Lys(UUU) and tRNA-Val(UAC), but not tRNA-Gly(UCC), have significantly en hanced steady-state levels in this mutant. Our data provide evidence for a variety of splicing factors and pathways in the chloroplast, some encoded b y nuclear and some by chloroplast genes, and possibly for a dual function o f some of these factors.