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Mutants of Sweetclover (Melilotus alba) Lacking Chlorophyll b^1,2

Studies on Pigment-Protein Complexes and Thylakoid Protein Phosphorylation

Department of Agricultural Biochemistry and School of Biological Sciences, University of Nebraska, Lincoln, Nebraska 68583-0718

Mutants of sweetclover (Melilotus alba) with defects in the nuclear ch5 locus were examined. Using thin-layer chromatography and absorption spectroscopy, three of these mutants were found to lack chlorophyll (Chl) b. One of these three mutants, U374, possessed thylakoid membranes lacking the three Chl b-containing pigment-protein complexes (AB-1, AB-2, and AB-3) while still containing A-1 and A-2, Chl a complexes derived from photosystems I and II, respectively. Complete solubilization and denaturation of the thylakoid proteins from this mutant revealed very little apoprotein from the Chl b-containing light-harvesting complexes, the major thylakoid proteins in normal plants. The normal and mutant sweetclover plants had active thylakoid protein kinase activities and numerous polypeptides were labeled following incubation with [-³²P]ATP. With the U374 mutant, however, there was very little detectable label co-migrating with the light-harvesting complex apoproteins on polyacrylamide gels. The Chl b-deficient chlorina-f2 mutant of barley (Hordeum vulgare) also had an active protein kinase activity capable of phosphorylating numerous polypeptides, including ones migrating with the same mobility as the light-harvesting complex apoproteins. These results indicate that the sweetclover mutants may be useful systems for studies on the function and organization of Chl b in thylakoid membranes of higher plants.

¹ Supported by United States Department of Agriculture, Science and Education Competitive Research Grant No. 83-CRCR-1-1346, and by the University of Nebraska-Lincoln Research Council. Paper No. 7604, Journal Series, Nebraska Agricultural Experiment Station.

² Inquiries concerning seed of sweet clover plants used in these studies should be addressed to: Dr. H. J. Gorz, USDA-ARS, or Dr. F. A. Haskins, Department of Agronomy, University of Nebraska, Lincoln, NE 68583-0915.

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