Separate photosensory pathways co-regulate blue light/ultraviolet-A-activated psbD-psbC transcription and light-induced D2 and CP43 degradation in barley (Hordeum vulgare) chloroplasts.
We studied the effects of spectral quality and fluence on the expression of several chloroplast-encoded photosynthesis genes and on the stability of their protein products in barley (Hordeum vulgare). During light-dependent chloroplast maturation, mRNA levels for psbD-psbC and psbA were maintained at higher levels compared with mRNAs encoding proteins for other photosynthesis functions (atpB, rbcL). Maintenance of psbD-psbC mRNA levels was accounted for by differential activation of the psbD-psbC light-responsive promoter by high-irradiance blue light and, secondarily, ultraviolet A (UV-A) radiation. Promoter activation was fluence dependent and required continuous illumination for 2 h at threshold fluences of 1.3 (blue light), 7.5 (white light), or 10 (UV-A) mumol m-2 s-1. From immunoblot analysis experiments, we showed that the psbD-psbC gene products D2 and CP43 undergo light-mediated turnover similar to light-labile D1. Other photosynthesis proteins such as the beta subunit of ATP synthase and the large subunit of ribulose-1,5-bisphosphate carboxylase were relatively stable. In the absence of protein synthesis, D2 degradation paralleled the degradation of D1 (relative half-lives, 9.5-10 h). CP43 decay was about half of D2 and D1 decay. In contrast with activation of the light-responsive promoter, the fluence-dependent degradation of D1, D2, and CP43 required 50- to 100-fold higher fluences of photosynthetically active white, red, blue, or UV-A irradiation. We interpret the different fluence and wavelength requirements to indicate that separate photosensory systems regulate activation of psbD-psbC transcription and turnover of D1, D2, and CP43. We propose that a blue light/UV-A photosensory pathway activates the psbD-psbC light-responsive promoter, differentially maintaining the capacity of mature chloroplasts to synthesize D2 and CP43, which are damaged and turned over in illuminated plants.
Department of Biochemistry and Biophysics, Texas A&M University, College Station 77843-2128.
D A
DA
Christopher
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JE
Mullet
D002736:Chloroplasts / Q000378:metabolism* / Q000528:radiation effects; D005786:Gene Expression Regulation / Q000528:radiation effects*; D017343:Genes, Plant / Q000528:radiation effects*; D001467:Hordeum / Q000235:genetics / Q000378:metabolism* / Q000528:radiation effects; D008027:Light; D010788:Photosynthesis / Q000235:genetics; D045322:Photosynthetic Reaction Center Complex Proteins / Q000096:biosynthesis* / Q000235:genetics / Q000528:radiation effects; D012333:RNA, Messenger / Q000096:biosynthesis / Q000378:metabolism; D014158:Transcription, Genetic / Q000187:drug effects; D014466:Ultraviolet Rays
D045322:Photosynthetic Reaction Center Complex Proteins; D012333:RNA, Messenger