A SINGLE NUCLEAR LOCUS IS INVOLVED IN BOTH CHLOROPLAST RNA TRANSSPLICING AND 3'-END PROCESSING

The complex maturation of the chloroplast psaA mRNA in Chlamydomonas r einhardtii requires the splicing of three separate exons in trans. At least 14 nuclear gene products and a small chloroplast RNA (tscA) seem to be involved in psaA trans-splicing. To further elucidate the mecha nisms that regulate trans-splicing in C. reinhardtii, we have analyzed four different nuclear mutants; two affected in trans-splicing of the psaA exons 1 and 2 (class C) and two blocked in both psaA trans-splic ing steps (class B). While mutants M18 (class C) and L118 (class B) ha ve a mature chloroplast tscA RNA, no such transcripts could be detecte d in mutants TR72 (class C) and HN31 (class B). Northern hybridization s indicate that the absence of mature tscA transcripts in HN31 and TR7 2 results from a defect in the processing of a tscA/chlN precursor tra nscript. Since the tscA locus has been shown to mediate psaA exon 1 an d 2 trans splicing, the nucleus-encoded factor affected in the class C mutant TR72 may be required for tscA 3' end maturation and only indir ectly for psaA exon 1 and 2 trans-splicing. Moreover, further genetic analysis revealed that the nuclear HN31 locus is involved in both matu ration of the tscA/chlN precursor and transsplicing of the psaA exons 2 and 3.