THE EFFECT OF LIGHT INTENSITIES ON THE TRANSCRIPT LEVEL OF PROTEINS INVOLVED IN PHOTOSYNTHESIS IN MUSTARD PLANTS

The influence of light quantity on the steady-state levels of plastid encoded transcripts was examined during the development of primary lea ves from mustard (Sinapis alba) plants. RbcL mRNA, petA mRNA, and psbA mRNA, which encode the large subunit (LSU) of Rubisco, Cyt f of the C yt b(6)/f complex, and D 1 protein of PSII were investigated in leaves grown under high-light (HL) or low-light (LL) conditions. Additionall y, the nuclear encoded 25 S rRNA was quantified. As a proportion of to tal RNA, the levels of 25 S rRNA, rbcL mRNA, petA mRNA, and psbA mRNA did not differ substantially in the HL versus LL plants. During leaf o ntogenesis, though, the proportion of psbA mRNA in total RNA increased drastically, in contrast to the other transcripts. The results calcul ated per unit fresh weight, dry weight, leaf area, and chlorophyll clo sely resembled each other and gave an idea of the absolute prevalence of the transcripts in the cell. The light regime had a remarkable infl uence on the absolute levels of 25 S rRNA, rbcL mRNA, and petA. mRNA: Paralleling total RNA content, the amounts of the three transcripts pe r unit fresh weight at the beginning of leaf development were, in HL p lants, more than twice the amounts found in LL plants. Furthermore, du ring leaf ontogenesis the decrease in total RNA, 25 S rRNA, rbcL mRNA, and petA mRNA was more pronounced in HL than in LL, while the psbA mR NA content per unit fresh weight was only slightly higher in HL than i n LL leaves and did not decline significantly during HL and LL leaf on togenesis. Comparison of the absolute levels of rbcL mRNA, petA mRNA, and psbA mRNA with the amounts of the corresponding proteins revealed that the prevalence of the transcripts contributes principally to the differences in the protein content in HL and LL leaves.