Structure and expression of the tobacco nuclear gene encoding the 33 kDa chloroplast ribonucleoprotein.

Tobacco chloroplasts contain a family of ribonucleoproteins (RNPs) which appear to be involved in mRNA processing and splicing in chloroplasts. We have characterized a new cDNA, 33k-6, potentially encoding a tobacco 33 kDa chloroplast RNP (cp33) homologue. This cDNA has a 78 bp insertion near the 3' end with respect to previously characterized cp33 cDNAs, leading to the creation of an alternative C-terminal sequence. The cp33 protein is encoded by a single-copy nuclear gene in Nicotiana sylvestris, which contains three introns. No typical TATA box is present in the upstream region of the gene. Multiple transcription start sites are often observed for promoters lacking TATA boxes, and have been suggested in the cp33 gene. Sequence comparison revealed that the 78 bp insertion in 33k-6 is derived from the third intron of the cp33 gene which is not removed during pre-mRNA splicing. Ribonuclease protection analysis showed that the processing of the third intron is slow compared to the other introns. A possible role for the partially spliced mRNA (cp33k-6) is discussed.