Investigation of the site of synthesis of chIcoroplastic enzymes of nitrogen metabolism by the use of heat‐treated 70S ribosomedeficient rye leaves

Abstract

The activities of the enzymes nitrate reductase (EC 1.6.6.1), nitrite reductase (EC 1.6.6.4), glutamine synthetase (EC 6.3.1.2), glutamate synthase (GOGAT; EC 1.4.7.1), glutamate‐oxaloacetate aminotransferase (EC 2.6.1.1), and glutamate dehydrogenase (EC 1.4.1.2) were compared in light‐grown green or etiolated leaves of rye seedlings (Secale cereale L. cv. Halo) raised at 22°C, and in the bleached 70S ribosome‐deficient leaves of rye seedlings grown at a non‐permissive high temperature of 32°C. Under normal permissive growth conditions the activities of most of the enzymes were higher in light‐grown, than in dark‐grown, leaves. All enzyme activities assayed were also observed in the heat‐treated 70S ribosome‐deficient leaves. Glutamine synthetase, glutamate synthase, and glutamate‐oxaloacetate aminotransferase occurred in purified ribosome‐deficient plastids separated on sucrose gradients. For glutamate‐oxaloacetate aminotransferase four multiple forms were separated by polyacrylamide gel electrophoresis from leaf extracts. The chloroplastic form of this enzyme was also present in 70S ribosome‐deficient leaves. It is concluded that the chloroplast‐localized enzymes nitrite reductase, glutamine synthetase, glutamate synthase and glutamate‐oxaloacetate aminotransferase, or their chloroplast‐specific isoenzyme forms, are synthesized on cytoplasmic 80S ribosomes.