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-encoded, plastid ribosomal protein L4 from spinach. The L4 protein belongs to the group of ribosomal proteins for which extra-ribosomal functions have been demonstrated in prokaryotes. In general, these functions are concerned with 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 analysed its role in both prokaryotic (E. coli) and plastid systems. We found that the plastid L4 protein can replace the E. coli L4 protein in the NusA-dependent 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.