The small subunit of the chloroplast enzyme ribulose bisphosphate carboxylase is synthesised as a precursor of higher molecular weight when poly A–mRNA from pea cytoplasmic polysomes is translated by wheat-germ ribosomes. This precursor is taken up into intact isolated chloroplasts, and cleaved to its final size in the absence of protein synthesis. It is deduced that the signal hypothesis does not apply to chloroplasts; an envelope carrier mechanism is proposed.

Synthesis and transport of the small subunit of chloroplast ribulose bisphosphate carboxylase

1
Antibodies raised against the 26000-Mr polypeptides of the light-harvesting chlorophyll a/b proteins of pea leaves specifically immunoprecipitated two 32000 -Mr 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 -Mr 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 -Mr precursor polypeptides could be detected in the soluble fraction (cytoplasm and stroma). However, low levels of the mature 26000 -Mr 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.

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1432-1033.1981.tb05486.x

Biosynthesis of the Light-Harvesting Chlorophyll a/b Protein

Illumination of dark-grown barley plants induces a massive insertion of the light-harvesting chlorophyll a/b protein into the developing thylakoid membrane. In addition to the onset of chlorophyll synthesis, light induces specifically the appearance of a prominent mRNA species which codes for a polypeptide of Mr 29500. This component was identified as a precursor of the apoprotein of the light-harvesting chlorophyll a/b protein. The precursor has an Mr larger than the authentic protein by approximately 4000. Studies of the chlorophyll-b-less mutant chlorina f2 of barley offer the first clue to the mechanism which controls the light-dependent mRNA formation. The induction of the mRNA coding for the aproprotein of the light-harvesting chlorophyll a/b protein does not seem to be linked directly to the assembly process of the light-harvesting structure and does not require chlorophyll b. It is proposed that light exerts its influence on the mRNA formation by a reaction which is different from the phototransformation of protochlorophyll(ide) to chlorophyll(ide).

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1432-1033.1978.tb12273.x

The plastid membranes of barley (Hordeum vulgare). Light-induced appearance of mRNA coding for the apoprotein of the light-harvesting chlorophyll a/b protein.

The molecular organization of chloroplast thylakoids.

Comparison of the composition of the energy-trapping apparatus of normal barley photo-synthetic membranes and those lacking chlorophyll b shows that not only is chlorophyll b absent from the mutant, but all constituents (chlorophyll a, chlorophyll b, carotenoids and the protein moiety) of the major chlorophyll-protein complex of normal higher plant chloroplast membranes are also missing; other chlorophyll-containing components, as far as can be ascertained, are essentially unaffected by the mutation. The nuclear gene which codes for the protein moiety of this complex is suggested as a possible site of the mutation in this barley mutant. This chlorophyll α/β-protein complex which accounts for such a large proportion of the chlorophyll and protein of the photosynthetic apparatus of higher plants is thus not essential for a plant to grow photosynthetically, and therefore a new name, light-harvesting chlorophyll-protein, is proposed for the complex, which was formerly termed the photosystem II chlorophyll-protein. The biosynthetic machinery in the mutant is unable to compensate for the loss of the light-harvesting chlorophyll-protein by adding extra chlorophylls into alternative sites in the membrane, thus the chlorophyll/P700 ratio in the mutant is two-thirds that of the normal plant. The light-harvesting chlorophyll-protein is the major location for chlorophyll b in higher plant membranes. The function of the complex is thought to be analogous to that of the algal biliproteins, i.e. light energy absorbed by the complex is fed preferentially, but not exclusively, to the photosystem II reaction-center or trap. It is reasoned that the complex exerts a strong influence on the organization of photosynthetic lamellae in higher plant chloroplasts by maintaining lamellae in contact with each other.

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1432-1033.1974.tb03250.x

Composition of the Photosynthetic Apparatus of Normal Barley Leaves and a Mutant Lacking Chlorophyll b

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1016/0014-5793%2872%2980763-4

S.G. Wildman

Papers

Nuclear DNA codes for the photosystem II chlorophyll-protein of chloroplast membranes.