INEFFICIENT RPL2 SPLICING IN BARLEY MUTANTS WITH RIBOSOME-DEFICIENT PLASTIDS

Analysis of transcript accumulation and splicing in plastids of four n uclear mutants of barley revealed that the ribosomal protein L2 (rpl2) gene transcripts containing a group II intron remained entirely unspl iced, whereas the intron of the ribosomal protein L16 (rpl16) gene (li nked with the rpl2 gene in the same operon) was removed in the mutant plastids. Also, the transcripts of other genes containing group II int rons (ribosomal protein S16 gene, rps16; NADH dehydrogenase ND2 gene, ndhB; cytochrome f gene, petD; and intron-containing reading frame 170 , irf170) and of the tRNA for leucine, trnL (UAA), possessing the only chloroplast group I intron, were found to be spliced. The mutants use d in this investigation are considered to be nonallelic; this excludes the possibility that a single nuclear gene is responsible for the imp aired splicing of rpl2 transcripts. The mutants, however, have a sever e deficiency in chloroplast ribosomes in common; this deficiency is ev ident from the lack of the essential ribosomal protein L2 and from an extremely low steady state level of plastid rRNAs. From these results, we conclude that a functioning translational apparatus of the organel le is a prerequisite for splicing of the chloroplast rpl2 class II int ron but not for splicing of at least five other group II intron-contai ning transcripts. This provides genetic evidence for a chloroplast DNA -encoded component (e.g., a maturase) involved in the splicing of rpl2 pre-mRNA.