Extra-ribosomal function(s) of the plastid ribosomal protein L4 in the expression of ribosomal components in spinach

We have previously characterised the cDNA corresponding to the nucleus-enco ded, plastid ribosomal protein L4 from spinach. The L4 protein belongs to t he group of ribosomal proteins for which extra-ribosomal functions have bee n demonstrated in prokaryotes. In general, these functions are concerned wi th the expression of ribosomal components. In order to analyse whether the plastid L4 protein might also have (an) extra-ribosomal function(s) we have produced the plastid L4 protein as a thioredoxin fusion protein and analys ed its role in both prokaryotic (E. coli) and plastid systems. We found tha t the plastid L4 protein can replace the E. coli L4 protein in the NusA-dep endent attenuation control of the E. coli S10 operon by stabilising stalled transcription complexes in a NusA-dependent reaction. In plastids, the L4 protein inhibits transcription of the rrn operon. Our results thus suggest extra-ribosomal function(s) for the plastid L4 protein in the expression of ribosomal components.