Stability of the Apoproteins of Light-Harvesting Complex I and II during Biogenesis of Thylakoids in the Chlorophyll b-less Barley Mutant Chlorina f2.

Transcription and translation of Lhc (cab) genes have been compared in the chlorina f2 mutant of barley (Hordeum vulgare) and its wild type to study the effect of chlorophyll b's absence on the regulation of assembly of the light-harvesting complexes (LHC). All tested genes were transcribed and the amount of their respective mRNAs increased rhythmically upon illumination of etiolated mutant plants. The synthesis of individual LHC apoproteins also had a rhythmic pattern when total leaf protein extracts were examined, whereas they increased gradually in the thylakoid. Only some LHC pigment-proteins present in wild-type thylakoids were found in mature mutant membranes. Thus, only the 25-kD (type 3) apoprotein of the three apoproteins of the major LHC IIb complex survived. The amount of the minor LHC II pigment-proteins was considerably reduced but not to zero. Photosystem I had some of the two LHC la apoproteins but had little of those of LHC lb. This was reflected in a shift of the 77-K emission maximum of whole leaves from 741 to 732 nm. It is concluded that the two largest LHC IIb and the LHC Ib apoproteins need chlorophyll b for stable integration into the membrane and that posttranslational regulation plays a major role in LHC assembly.