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Synthesis of a Putative c-Type Cytochrome by Intact, Isolated Pea Chloroplast ¹

Department of Botany, University of California at Davis, Davis, California 95616

In addition to chlorophyll-protein complexes, other proteins were labeled when isolated developing pea (Pisum sativum L.) chloroplasts were incubated with [¹⁴C]-5-aminolevulinic acid. The major labeled band (M_r = 43 kilodaltons by lithium dodecyl sulfate-polyacrylamide gel electrophoresis) was labeled even in the presence of chloramphenicol. Heme-dependent peroxidase activity (as detected by the tetramethyl benzidine-H₂O₂ stain) was not visibly associated with this band. The radioactive band was stable to heat, 5% HCl in acetone, and was absent if the incubation with [¹⁴C]-5-aminolevulinic acid was carried out in the presence of N-methyl protoporphyrin IX dimethyl ester (a specific inhibitor of ferrochelatase). Organic solvent extraction procedures for the enrichment of cytochrome f from chloroplast membranes also extracted this unknown labeled product. It was concluded that this labeled product was probably a c-type cytochrome; however, the possibility that it might be a protein containing a covalently linked linear tetrapyrrole was not ruled out.

¹ Supported by National Science Foundation Grants PCM 81-08538 and DMB-8415321.