Regulation of accumulation of the major thylakoid polypeptides in Chlamydomonas reinhardtii y-1 at 25 degrees C and 38 degrees C.

Abstract

The amount of messenger RNA (mRNA) for polypeptides of the chlorophyll a/bprotein complex of thylakoid membranes in etiolated and greening cells of Chlamydomonas reinhardtii y-1 was examined by immunoprecipitation and electrophoresis of products of in vitro translation to determine at which stage production of these polypeptides is regulated. Cells grown 4 d in the dark at 25°C contained small amounts of translatable mRNA for the major membrane polypeptides. Exposure of these etiolated cells to light, under conditions in which the membrane polypeptides accumulated, resulted in a significant increase in the quantity of the mRNA. In contrast, when etiolated cells were incubated for 1-2 h in the dark at 38°C, translation assays indicated that mRNA for the membrane polypeptides became abundant. Moreover, the quantity of the mRNA did not increase when these cells subsequently were exposed to light. Therefore, at 38°C the cellular level of the polypeptides is not regulated by synthesis of mRNA. The in vitro synthesized polypeptides, which were precipitated with antibodies prepared against the purified thylakoid polypeptides, had apparent molecular weights of 31,500 and 30,000. The corresponding immunoprecipitated polypeptides made in vivo had apparent molecular weights of 29,500 and 26,000. Thus, the membrane polypeptides are made as precursors.No net accumulation of the polypeptides occurred in cells in the dark at 38°C, but immunoreactive polypeptides the size of the mature membrane components were labeled during incubation of cells with [14C]acetate in the dark. These results indicated that the mRNA was translated in the dark, but since the polypeptides did not accumulate, the products of translation were probably degraded. We conclude from our experiments that at 25°C production of the polypeptides is regulated by the level of translatable mRNA in the cells. At 38°C, however, the accumulation of the polypeptides is controlled by posttranslational processes.

Kinetics of Chlorophyll Accumulation and Formation of Chlorophyll-Protein Complexes during Greening of Chlamydomonas reinhardtii y-1 at 38°C

“…The immediate and rapid increase in the amount of Chl with onset of exposure of the algal cells to light is correlated with the prior accumulation of mRNA for LHCPs (14). In this regard, this situation is similar to those higher plants in which a pulse of red light induces synthesis of LHCP mRNA through activation of phytochrome, thereby eliminating the lag in Chl accumulation (17).…”

“…This complex in higher plants recently was described as an aggregate of three polypeptides (LHCPs) each associated with eight Chl a molecules, seven Chl b molecules, and several carotenoids (7). Degreened C. reinhardtii y-cells contain a barely detectable level oftranslatable mRNA for the LHCPs at 25°C (14,22) but develop a full complement of these mRNAs during a 2-h incubation in the dark at 38°C (14). As shown by the response to phenanthro-lines, these cells are fully capable of synthesizing Chlide b in the dark (3)(4)(5).…”

Three light-inducible heat shock genes of Chlamydomonas reinhardtii.

“…Heat-induced accumulation of mRNA species encoding major polypeptides of the thylakoid membrane has been observed in etiolated cells of the C. reinhardtii mutant y-1 exposed to 38°C in the dark (21,22). In our experiments with Chlamydomonas wild-type cells grown in the light, heat shock had no effect on levels of rbcS2-encoded mRNA (Fig.…”

Origin of Thylakoid Membranes in Chlamydomonas reinhardtii y-1 at 38°C

“…The lag reflects, at least in part, the kinetics of induction by light of expression of genes encoding LHCPs (19). However, expression of these genes is also induced in the dark by higher temperatures (12,16). After a suspension of yellow cells is incubated at 38°C for 1 to 2 h, Chl and LHCPs accumulate without a lag and at linear rates when the cells are exposed to light (13,20).…”