C14 Amino Acid Incorporation by Spinach Chloroplast Preparations

Abstract
Apparent incorporation of amino acids into protein by isolated spinach chloroplasts was found to be due to contaminating bacteria which are sedimented together with the chloroplasts during their initial isolation. At least 75% of the labeled protein is separable from the chloroplasts in a subsequent density gradient centrifugation. Conditions which inhibit incorporation (brief heating at 50[degree]C: hypochlorite treatment of the leaf before homogenizing; addition of chloramphenicol, puromycin, or streptomycin) also inhibit growth of the contaminating bacteria. An apparent stimulation by light is found in the present experiments to be observable only under anaerobic conditions. The light is shown to cause chloroplasts to evolve some O2 which in turn is needed for protein incorporation by the contaminating bacteria. It is concluded that certainly not in the present experiments, and possibly not in others reported to date, has it been demonstrated unequivocally that isolated chloroplasts are capable of amino acid incorporation.
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