A transcription map of the pea chloroplast genome

A set of 53 cloned pea chloroplast DNA fragments representing approximately 90% of the chloroplast genome was used to probe Northern blots of total pea RNA, resulting in a nearly complete chloroplast transcription map. Similar analyses were performed for RNAs extracted from pea seedlings grown under several different light regimes. We have found that at least 85 kb of the 120 by pea chloroplast genome is represented as detectable transcripts. For many regions of the genome, we have detected multiple overlapping transcripts including both small, gene-sized RNAs and large transcripts covering entire gene clusters. All transcripts detected were more abundant (as a fraction of total cellular RNA) in light grown plants than in plants entirely in the dark. However, larger transcripts were generally more abundant in plants that had been exposed to only 24 h of white light (after germination in the dark) than in plants grown in continuous light. This study indicates that chloroplast genes are often grouped into multigene transcriptional units which can be cotranscribed, and that light-stimulated plastid development involves changes in the relative abundance of the overlapping RNAs of different length that result from transcription of these genes or gene clusters.