| Abstrakt: |
Higher plant plastid genomes encode rRNAs, tRNAs, and protein subunits of the RNA polymerase, ribosomes, and the photosynthetic apparatus which vary over 1000-fold in abundance. Quantitative analysis of transcription and RNA levels was carried out on 15 plastid genes which are located in 14 different transcription units covering 50% of the barley plastid genome. Transcription of 16S rRNA, trnfM-trnG, and trnK was high relative to most other plastid genes. Transcription of trnfM-trnG was 5 times greater than trnK indicating that differences in tRNA levels in plastids could be due, in part, to differences in transcription. Among the protein coding genes, mRNA levels varied over 900-fold and transcription over 300-fold. The gene showing the lowest transcription rate and mRNA level, rpoB, is located in a gene cluster which encodes subunits of the plastid RNA polymerase (rpoB-rpoC1-rpoC2). RpoA, which encodes the alpha subunit of the RNA polymerase, was located in a gene cluster encoding ribosomal proteins (rpl23, rps19, rpl16) and infA. RNA from this gene cluster is 30-fold more abundant than rpoB mRNA, suggesting that expression of rpoA is regulated at the level of translation or protein stability. Polycistronic operons encoding subunits of the photosynthetic apparatus (psbB-psbH-petB-petD; psbK-psbI-psbD-psbC; atpB-atpE; psaA-psaB) had higher transcription rates and correspondingly higher mRNA levels than genes which encode ribosomal proteins or RNA polymerase subunits. RbcL and psbA, which are located in separate transcription units, exhibited the highest transcription rates and mRNA levels. Correspondence between transcription rate, mRNA level, and protein abundance indicates that transcription is a primary determinant of barley plastid gene expression. In addition, a 30-fold variation in predicted mRNA stability was observed which further increases the dynamic range of plastid mRNA abundance. |
