Intergenic cleavages, intron splicing, and editing of primary transcripts of the plastid ndhH-D operon produce multiple overlapping RNAs, of which the most abundant by far is the monocistronic 400-nucleotide mRNA of psaC (encoding the PsaC protein of photosystem I), in contrast with the low level of transcripts of the six ndh genes. Like other plastid operons containing genes for functionally unrelated proteins, the contrasting accumulation of ndh and psaC transcripts provides a model to investigate the mechanisms of the post-transcriptional control of gene expression, a feature of chloroplast genetic machinery, with a minimum of interference by transcriptional control. In leek (Allium porrum L), the ndhD transcript (which follows the psaC gene and ends the ndhH-D operon) requires C 3 U editing to restore its start codon and may be used as a marker for the processing of psaC and ndhD transcripts. By determining the editing state and 5 end sequences of specific transcripts, we demonstrated that stable monocistronic psaC mRNA results from downstream cleavages in the ndhD sequence, which renders non-functional ndhD transcripts as by-products. Alternative psaC-ndhD intergenic cleavages produce complete mRNAs for both genes, but only take place in precursors containing editing-restored ndhD start codons. Hence, post-transcriptional control acts by promoting the ndhD cleavage alternative, which allows the accumulation of psaC mRNA at the expense of ndhD mRNA levels.Many chloroplast genes are expressed as polycistronic transcription units that are processed to complex sets of overlapping RNAs through steps controlled by nuclear encoded factors (1, 2). A lot remains to be learned about factors, intermediates, and the order in which transcript processing occurs as well as the mechanisms responsible for the accumulation of specific mRNAs (frequent in operons containing genes for functionally unrelated proteins). This last is a feature of chloroplast genetic machinery in which post-transcriptional processing controls the differential expression of specific genes within the same operon.The NdhH-D operon provides a model to investigate posttranscriptional control of gene expression, because it includes six ndh genes whose protein products are present at low levels in chloroplasts, together with the psaC gene, encoding the PsaC protein (9-kDa Fe-S subunit VII) of the abundant photosystem I. The polypeptides encoded by ndh genes are part of the thylakoid Ndh complex that has been purified from peas (3) and barley (4). The photosystem I/Ndh ratio in chloroplasts is estimated at 200. Accordingly, the level of monocistronic psaC mRNA is estimated to be around 2 orders of magnitude higher than that of ndh genes (5-8).The NdhH-D operon includes (in this order) ndhH, ndhA, ndhI, ndhG, ndhE, psaC, and ndhD genes (9) and produces a complex pattern of transcripts resulting from intergenic cleavages, intron splicing (within ndhA), and C 3 U editing at several specific sites (usually in ndhA and ndhD genes). The requirement of intron spl...