Controls on chlorophyll synthesis in barley.

In 7- to 10-day-old leaves of etiolated barley (Hordeum vulgare), all of the enzymes that convert delta-aminolevulinic acid to chlorophyll are nonlimiting during the first 6 to 12 hours of illumination, even in the presence of inhibitors of protein synthesis. The limiting activity for chlorophyll synthesis appears to be a protein (or proteins) related to the synthesis of delta-aminolevulinic acid, presumably delta-aminolevulinic acid synthetase. Protein synthesis in both the cytosol and plastids may be required to produce nonlimiting amounts of delta-aminolevulinic acid. The half-life of a limiting protein controlling the synthesis of delta-aminolevulinic acid appears to be about 1(1/2) hours, when determined with inhibitors of protein synthesis. Acceleration of chlorophyll synthesis by light is not inhibited by inhibitors of nucleic acid synthesis, but is inhibited by inhibitors of protein synthesis. A model for control of chlorophyll synthesis is proposed, based on a light-induced activation at the translational level of the synthesis of proteins forming delta-aminolevulinic acid, as well as the short half-life of these proteins. Evidence is presented confirming the idea that the holochrome on which protochlorophyllide is photoreduced to chlorophyllide functions enzymatically.