Synthesis of soluble, thylakoid and envelope polypeptides by isolated chloroplasts of Sorghum vulgare

Abstract

Isolated chloroplasts from the seedlings of Sorghum vulgare leaves incorporated 14C-labelled amino acids into soluble and membrane-bound products, using light as the sole energy source. The labelled chloroplasts were lysed osmotically and fractionated on a discontinuous gradient of sucrose into the soluble, thylakoid and envelope membrane fractions. About 24% of the total radioactivity in the chloroplasts was recovered in the soluble fraction, 66% in the thylakoid membranes and less than 1% in the envelope membranes. The products of protein synthesis in the different fractions, as well as in the whole chloroplasts were analyzed by electrophoresis on polyacrylamide gels in the presence of sodium dodecyl sulfate. There were three zones of radioactivity in the gels of the soluble fraction. The thylakoid membranes contained nine labelled polypeptides, the most prominent ones having the molecular weights of about 66 000, 56 000 and 27 000. The envelope membranes contained a major radioactive component of molecular weight of about 54 000 and two other minor components.