Biosynthesis of the Light‐Harvesting Chlorophyll a/b Protein

Abstract

1 Antibodies raised against the 26000‐M_r polypeptides of the light‐harvesting chlorophyll a/b proteins of pea leaves specifically immunoprecipitated two 32000 ‐M_r polypeptides synthesized when pea leaf poly(A)‐containing RNA was translated in vitro. On the basis of immunochemical relatedness and by comparison of their partial tryptic digestion products, the 32000 ‐M_r products formed in vitro are identified as precursors to the authentic polypeptides of the light‐harvesting chlorophyll a/b complex. 2 The specificity of the immunoprecipitation permitted the development of an assay for the cellular levels of translationally active light‐harvesting protein mRNA in plants exposed to different light regimes. Low levels of the mRNAs were detectable in dark‐grown plants. Exposure to continuous illumination caused these levels to increase by at least ten‐fold and led to the appearance of large quantities of the light‐harvesting chlorophyll a/b complex. In plants exposed to intermittent illumination (2 min of white light every 2 h for 2 days), the light‐harvesting complex did not accumulate, although levels of mRNA specifying the polypeptides of the complex were high (50% of those in continuously illuminated plants). 3 Messenger RNAs encoding the light‐harvesting proteins were detected in polysomes of intermittently illuminated leaves. These polysomes were active in a wheat‐germ 100000 ×g supernatant ‘run‐off’ system, to form light‐harvesting protein precursors, under conditions when only nascent polypeptide chains initiated in vivo were elongated and terminated. These results demonstrate that the inability of intermittently illuminated leaves to accumulate the light‐harvesting proteins is not due to a selective inhibition of the translation of the corresponding mRNAs. 4 Intermittently illuminated leaves were labelled with [³⁵S]methionine in darkness, and incorporation of radioisotope into the light‐harvesting proteins and their precursors was assayed immunologically. No pool of untransported or unprocessed 32000 ‐M_r precursor polypeptides could be detected in the soluble fraction (cytoplasm and stroma). However, low levels of the mature 26000 ‐M_r polypeptides were detected in the membrane fraction. It is concluded that the newly synthesized light‐harvesting chlorophyll a/b proteins fail to accumulate in intermittently illuminated leaves because they undergo rapid turnover. The site of light‐harvesting protein breakdown is probably the thylakoid membrane, and the cause of breakdown is probably the absence of chlorophyll a and chlorophyll b molecules that are required for eventual stabilization of the proteins within the photosynthetic membrane.