The presence of DNA in ribosome-deficient plastids of heat-bleached rye leaves.

In leaves of rye seedlings (Secale cereale L.) grown at 32 degrees C the formation of plastid (70-S) ribosomes is specifically prevented. The resulting plastid-ribosome-deficient leaves can be used as a suitable system to identify chloroplast proteins which are translation products of cytosolic (80-S) ribosomes. The ribosome deficiency in plastids is accompanied by a bleaching of the leaves in light. In experiments aimed at finding the primary heat-sensitive event leading to ribosome deficiency the DNA of rye chloroplasts has been identified. Its properties are similar to those of chloroplast DNAs from other higher plants. The ribosome-deficient plastids isolated from heat-bleached rye leaves contained a DNA species which was indistinguishable from that of chloroplasts with regard to buoyant density in CsCl equilibrium gradients, reassociation properties and fragment patterns obtained upon cleavage by restriction endonucleases. Its quantity was comparable to that of chloroplast DNA of green leaves grown at a permissive temperature (22 degrees C). These results suggest that, unlike the effect in heat-bleached Euglena strains, lack of chloroplast DNA cannot be considered as the reason for the primary effect of high temperature in rye leaves but steps in the biosynthetic pathway of plastid ribosomes themselves must be affected more directly.