Knock-out of the plastid ribosomal protein L11 in Arabidopsis: effects on mRNA translation and photosynthesis

The prpl11-1 mutant of Arabidopsis thaliana was identified among a collecti on of T-DNA tagged lines on the basis of a decrease in the effective quantu m yield of photosystem II. The mutation responsible was localized to Prpl11 , a single-copy nuclear gene that encodes PRPL11, a component of the large subunit of the plastid ribosome. The amino acid sequence of Arabidopsis PRP L11 is very similar to those of L11 proteins from spinach and prokaryotes. In the prpl11-1 mutant, photosensitivity and chlorophyll fluorescence param eters are significantly altered owing to changes in the levels of thylakoid protein complexes and stromal proteins. The abundance of most plastome tra nscripts examined, such as those of genes coding for the photosystem II cor e complex and RbcL, is not decreased. Plastid ribosomal RNA accumulates in wild-type amounts, and the assembly of plastid polysomes on the transcripts of the rbcL, psbA and psbE genes remains mainly unchanged in mutant plants , indicating that lack of PRPL11 affects neither the abundance of plastid r ibosomes nor their assembly into polysomes. However, in vivo translation as says demonstrate that the rate of translation of the large subunit of Rubis co (RbcL) is significantly reduced in prpl11-1 plastids. Our data suggest a major role for PRPL11 in plastid ribosome activity per se, consistent with its location near the GTPase-binding centre of the chloroplast 50S ribosom al subunit. Additional effects of the mutation, including the pale green co lour of the leaves and a drastic reduction in growth rate under greenhouse conditions, are compatible with reduced levels of protein synthesis in plas tids.