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The development of increased activities of ribulosediphosphate carboxylase (EC 4.1.1.39) and of phosphoribulokinase (EC 2.7.1.19) in greening bean leaves was completely inhibited by D-threo chloramphenicol but unaffected by L-threo chloramphenicol. This indicates that these enzymes are synthesized by the ribosomes of the developing plastids. A different mechanism appears to be responsible for the development of activity of NADP-dependent triosephosphate dehydrogenase (EC 1.2.1.13) where the D-threo isomer gave 45% inhibition and the L-threo isomer gave 18% inhibition. Thus both specific (D-threo isomer) and unspecific (both isomers) inhibition occurred. It is suggested that the development of NADP-dependent triosephosphate dehydrogenase activity may result from the allosteric activation, in the plastids, of the NAD-dependent enzyme (Müller et al., 1969) which has been synthesized by cytoplasmic ribosomes. Neither isomer inhibited the development of five other enzymes of the photosynthetic carbon cycle namely ribosephosphate isomerase (EC 5.3.1.6), phosphoglycerate kinase (EC 2.7.2.3), triosephosphate isomerase (EC 5.3.1.1), tructosediphosphate aldolase (EC 4.1.2.13) and transketolase (EC 2.2.1.1), but there was a significant stimulation of the activity of transketolase by D-threo chloramphenicol.

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SUMMARYFor 14-day-old dark-grown beans a quantitative study of plastid membrane formation has been made for light-induced chloroplast development in the primary leaves. During the first 10 h of illumination, thylakoid formation resulted from the rearrangement of the membranes of the prolamellar body. De novo thylakoid formation commenced between 10 and 15 h after the beginning of illumination and resulted, in the course of 160 h of illumination, in a ten-fold increase in thylakoid membrane. During the course of greening no cell division occurred in the mesophyll cells, plastid expansion took place in all of the mesophyll cells but plastid division only occurred in the spong)' mesophyll cells to result in an 11 % increase in the plastid number of the whole leaf during the first 18 h of illumination.When leaves were supplied with D-Mreo-chloramphenicol during greening there was a severe inhibition of thylakoid formation, vesicles were formed in the plastid stroma and rather fewer and larger grana than normal were produced. However, the proportion of thylakoid material in grana was normal. As L-^Arco-chloramphenicol yielded exactly normal chloroplasts it is concluded that thylakoid synthesis was inhibited by D-