Translational control of chloroplast protein synthesis during senescence of barley leaves

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Abstract
A light‐driven homologous system for the translation of chloroplast mRNA has been prepared from chloroplasts isolated from senescent barley leaves and treated with micrococcal nuclease. We have investigated the potential of this system for studies on translational specificity. Chloroplast mRNAs from senescent and kinetin‐treated leaves have been translated in the homologous system and the products have been compared with the polypeptides synthesized by intact chloroplasts isolated from senescent and kinetin‐treated leaves. Isolated intact chloroplasts from senescent barley leaves specifically synthesized polypeptides of: 80,66,58,50,48 and 35 kDa but did not synthesize the 54 kDa large subunit of ribulose‐1, 5‐bisphosphate carboxylase oxygenase. Chloroplasts isolated from leaves treated for 18 h with kinetin specifically synthesized polypeptides of: 85,75 and 54 kDa. Several polypeptides (40,32,25 and 18 kDa) are synthesized by chloroplasts of both senescent and kinetin‐treated leaves. The homologous translation system derived from senescent chloroplasts synthesized only senescence polypeptides whether it was supplied with chloroplast mRNA from senescent leaves or leaves treated with kinetin for 18 h. The homologous translation system derived from chloroplasts of kinetin‐treated leaves synthesized either senescence polypeptides or kinetin‐induced polypeptides depending on the source of chloroplast mRNA, senescent leaves or leaves treated with kinetin, respectively. The results indicate that kinetin controls chloroplast gene expression in senescent leaves at both the transcriptional and translational level.