Light Effects on the Synthesis of Ribulose-1,5-Bisphosphate Carboxylase in Lemna gibba L. G-3

Abstract

Placing light-grown Lemma gibba L. G-3 into the dark results in a changed pattern of protein synthesis. Although the amount of protein in the tissue and the over-all rate of incorporation of I8'Slmethionine into protein does not significantly decline during four days of darkness, the rate of synthesis of three polypeptides declines dramatically. One of these polypeptides is the chlorophyil a/b-binding protein and the two others are the large and small subunits of ribulose-1,5-bisphosphate carboxylase. The changed rates of synthesis of the two subunits were examined after transitions of plants from light to dark and dark to light. The in vivo synthesis of both subunits, while declining to a low level during four days of darkness, increases rapidly upon retuming the plants to white ight. In addition, the level of poly(A) mRNA coding for the precursor pobpeptide of the small subunit of the enzyme falls to a low level in the dark and increases rapidly in response to white light. The increase in translatable mRNA for the small subunit is rapid enough to account for a major part of the increased synthesis of this subunit.Light affects the synthesis of a number of plant enzymes. One such enzyme is RuBP2 carboxylase, the major soluble protein in chloroplasts. One molecule of this enzyme (mol wt -550,000) contains eight large and eight small subunits. The large subunit is coded and synthesized in chloroplasts, whereas the small subunit is coded in the nucleus. This small subunit is synthesized in the cytoplasm as a precursor polypeptide from which a 4,000-to 8,000-dalton portion (depending on the species) is cleaved from the N-terminal end before it is transported into and assembled in the chloroplasts into the holoenzyme (9, 15). We have extended our earlier report (32) on the effects of white light and darkness on the level of translatable mRNA for the small subunit and on the rate of synthesis of both subunits of the enzyme in Lemna by looking at the time course of these changes and characterizing them in more detail through the use of immunoprecipitation.Studies with a number of species of higher plants have shown that the activity of RuBP carboxylase increases during greening of etiolated tissue (e.g. 14, 23) and phytochrome action is involved in regulation of this increase in some species (5,12

Biosynthesis of P700-Chlorophyll a Protein Complex, Plastocyanin, and Cytochrome b6/f Complex

“…Haslett and Cammack (8) (20,26). This protein is present in the etiolated pea leaves and the level increases 15-fold during 72 h greening period (20).…”

“…The biosynthesis of light-harvesting Chl a/b protein associated with PSII and ribulose 1,5-bisphosphate carboxylase has been extensively studied (2,5,20,26). The increased formation of these proteins occurring upon illumination of dark-grown plants is partly due to the increase in levels of the corresponding mRNAs (2,5,20,26).…”

White Light Effects on the mRNA for the Light-Harvesting Chlorophyll a/b-Protein in Lemna gibba L. G-3

“…The methods referred to below have been described in greater detail in our earlier work (24)(25)(26). L. gibba L: G-3 were grown aseptically under continuous light (24).…”

“…We have shown (24,26) that in L. gibba the effects of light and dark on the synthesis of RuBP carboxylase are due in part to their effect on the level of translatable mRNA coding for the small subunit of the enzyme. The amount of translatable mRNA coding for the precursor of the small subunit of RuBP carboxylase decreases relative to other mRNAs when light-grown plants are moved into the dark.…”

“…One ofthe major cellular events which is affected by light is the synthesis and development of an active photosynthetic apparatus. In Lemna gibba, an aquatic plant, the synthesis of the major soluble chloroplast protein, RuBP2 carboxylase, can be regulated by light (24,26). There are many other examples of species of higher plants in which the synthesis of this protein is affected by light (10,11,12,15,19).…”