Photosystem I-deficient Chlamydomonas reinhardtii mutants M18 and F27 lack mature mRNA for the psaA1 gene of photosystem I but accumulate unspliced and partially spliced psaA1 transcripts. Analysis of these transcripts indicate that 1) exon 1 of psaA1 is transcribed separately from exon 2, 2) exon 2 is cotranscribed with the psbD gene, and 3) at least two nuclear gene products are required for exon 1-exon 2 trans-splicing. In addition, analysis of chloroplast transcription in permeabilized

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... shows that exon 3 is near the 5' end of a separate transcription unit, indicating that exon 2-exon 3 ligation also occurs by a trans-splicing mechanism.