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 defective 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 albostrians mutation causes the loss of plastid ribosomes resulting secondarily in a disruption of splicing of all subgroup IIA introns in the chloroplast. Thus MatK, the only putative chloroplast intron-specific maturase of higher plants, might have evolved to function in splicing of multiple introns. We show that in the case of tRNA-Ala(UGC)the first step of splicing is affected, as suggested by the absence of lariat molecules. Thus the plastid-encoded splicing factor lacking in albostrians must participate in the formation of the catalytically active structure. In contrast, a mutation in the nuclear gene crs1 prevents splicing of only one intron but causes specific additional effects as precursor transcripts for tRNA-Ile(GAU), tRNA-Ala(UGC), tRNA-Lys(UUU)and tRNA-Val(UAC), but not tRNA-Gly(UCC), have significantly enhanced steady-state levels in this mutant. Our data provide evidence for a variety of splicing factors and pathways in the chloroplast, some encoded by nuclear and some by chloroplast genes, and possibly for a dual function of some of these factors.