Biosynthesis of chlorophyll a/b-binding polypeptides in wild type and the chlorina f2 mutant of barley.

Thylakoid membrane polypeptides of wild type barley and the chlorina i2 mutant, which lacks chlorophyll b, were analyzed by two-dimensional polyacrylamide gel electrophoresis. The mutant lacks o r is greatly deficient in polypeptides 14 (M,. = 27,000), 15 (Mr = 25,000), 20 (Mr = 22,000), 24 (Mr = 21,000), and 25 (Mr = 20,000) and contains reduced amounts of polypeptide 16 (Mr = 24,000). Polypeptides 14, 15, and 16 are major components of the light-harvesting chlorophyll a/b-protein complex (Burke, J. J., Steinback, K. E., and Arntzen, C. J. (1979) Plant Physiol. 63, 237-243), while the other polypeptides are constituents of the photosystem I chlorophyll antennae (Mullet, J. E., Burke, J. J., and Arntzen, C. J. (1980) Plant Physiol. 65, 814-822). To determine the molecular basis of the membrane lesion, we compared (i) poly(A) RNA translation products of wild type and the mutant and (ii) the ability of chloroplasts from both plants to import polypeptides in vitro. The mutant contains mRNA for membrane polypeptides 14, 15, 16, 20, and 25 and these polypeptides are indistinguishable from their wild type counterparts as determined by the uptake assay. Furthermore, chloroplasts prepared from the mutant are capable of importing newly synthesized polypeptides, either from wild type or the mutant, and incorporating them into thylakoid membranes. Thus, chlorophyll b is not needed for the synthesis, transport, processing, or thylakoid insertion of the polypeptides in question. The in vitro results are corroborated by results obtained from in vivo labeling of excised leaves. All of the polypeptides that are either missing from or deficient in the mutant thylakoids are in fact synthesized in vivo. Taken together, these results strongly suggest that the polypeptides turn over rapidly in the absence of chlorophyll b. Thus, chlorophyll b may be needed to stabilize thylakoid membrane polypeptides which bind this photosynthetic pigment.