Neither the availability of D2 nor CP43 limits the biogenesis of PSII in tobacco

The pathway of photosystem II assembly is well understood and multiple auxiliary proteins supporting it have been identified. By contrast, little is known about rate-limiting steps controlling PSII biogenesis. In the green alga Chlamydomonas reinhardtii, biosynthesis of the chloroplast-encoded D2 reaction center subunit (PsbD) limits PSII accumulation. To determine the importance of D2 synthesis for PSII accumulation in vascular plants and elucidate the contributions of transcriptional and translational regulation, the 5-untranslated region of psbD was modified via chloroplast transformation in tobacco. A drastic reduction in psbD mRNA abundance resulted in a strong decrease of PSII content, impaired photosynthetic electron transport, and retarded growth under autotrophic conditions. Overexpression of the psbD mRNA also increased transcript abundance of psbC (the CP43 inner antenna protein), which is co-transcribed with psbD. Because translation efficiency remained unaltered, translation output of pbsD and psbC increased with mRNA abundance. However, this did not result in increased PSII accumulation. The introduction of point mutations into the Shine-Dalgarno-like sequence or start codon of psbD decreased translation efficiency without causing pronounced effects on PSII accumulation and function. These data show that neither transcription nor translation of psbD and psbC are rate-limiting for PSII biogenesis in vascular plants, and that PSII assembly and accumulation in tobacco are controlled by different mechanisms than in Chlamydomonas. One sentence summaryPSII biogenesis in tobacco is neither limited by transcript accumulation nor translation of psbD and psbC.