Photochrome-mediated regulation of messenger RNAs for the small subunit of ribulose 1, 5-bisphosphate carboxylase and the light-harvesting chlorophyll a/b-protein in Lemna gibba.

Brief red illumination (10 min/8 hr) of Lemna gibba L. G-3 growing heterotrophically in the dark increases the growth of the plants and results in a substantial increase in the levels of mRNA for two major chloroplast polypeptides. These two nuclear-coded polypeptides are the light-harvesting chlorophyll a/b-protein, an intrinsic thylakoid membrane protein, and the small subunit of the stromal enzyme ribulose 1, 5-bisphosphate carboxylase [RuP2; 3-phospho-D-glycerate carboxylyase (dimerizing), E.C.4.1.1.39]. The effect of 10 min red illumination on the dark growth of the plants is reversed by immediate far-red illumination, but the effect on the mRNA levels is not. However, this latter response can be reversed by far-red light if the time between the beginnings of the red and far-red illumination is reduced to one minute. Thus phytochrome is the photoreceptor mediating both responses, and the effect on amounts of the translatable mRNAs has a remarkably short escape time.As expected from the high level of its mRNA in the plants grown in the dark with intermittent red illumination, the small subunit of ribulose 1, 5-bisphosphate carboxylase is synthesized in these plants and accumulates without further illumination. However, despite the relatively high levels of mRNA for the chl a/b-protein in the dark grown plants, this protein does not appear to be synthesized and inserted into the thylakoid membranes until the plants are transfered into white light. Thus, the normal synthesis of this protein must require light for some post-transcriptional process.