Transcription of the gene encoding the 51 kd chlorophyll a-apoprotein of the photosystem II reaction centre from spinach

Summary

Transcription of the plastid gene encoding the 51 kd chlorophyll a-conjugated polypeptide, which is presumed to carry the P₆₈₀ reaction centre chlorophyll of photosystem II, has been studied both in vivo and in vitro in spinach. The gene is part of a polycistronic transcription unit whose expression involves both transcriptional and post-transcriptional regulation. Two distinct transcripts have been detected in RNA from chloroplasts as well as from etioplasts which start 55 and 175 nucleotides upstream of the protein-coding region. In vitro transcription of the gene in a homologous chloroplast extract yields only the “-175” terminus and only this 5′ end is preceded by typical prokaryotic promoter elements. The “-55” terminus lacks similar sequences but starts within a stem-loop structure. This suggests that the transcription initiation site of the gene is located 175 nucleotides upstream of the translational start codon and that the latter terminus is the result of post-transcriptional processing of the primary transcript. The start site for another transcription unit which is read in the opposite direction has been mapped 210 bp proximal to the “-175” terminus. The nature of its product is unknown. It is likely that the intergenic region contains all sequence elements necessary for the accurate transcription of both genes.