Subpopulations of chloroplast ribosomes change during photoregulated development of Zea mays leaves: Ribosomal proteins L2, L21, and L29

Seedlings grown in darkness, i.e., etiolated seedlings, lack chlorophyll an d most other components of the photosynthetic apparatus. On illumination, t he plastids become photosynthetically competent through the production of c hlorophylls and proteins encoded by certain chloroplast and nuclear genes. There are two types of photosynthetic cells in leaves of the C4 plant maize : bundle sheath cells (BSC) and adjacent mesophyll cells (MC), Some protein s of the maize photosynthetic machinery are solely or preferentially locali zed in MC and others in BSC. A particular gene may be photoregulated up in one cell type and down in the other, Transcripts of the nuclear gene rpl29, encoding the chloroplast ribosomal protein L29, increase in abundance abou t 17-fold during light-induced maturation of plastids, There is about 1.5 t imes more L29 protein in ribosomes of greening leaves than in ribosomes of unilluminated leaves; the L29 contents of MC and BSC are about the same. Ho wever, L21 is present about equally in plastid ribosomes of unilluminated a nd illuminated seedlings. In contrast to both L29 and L21, the fraction of the ribosome population containing L2 is about the same in MC and BSC of et iolated leaves but, on illumination, the proportion of the ribosome populat ion with L2 increases in BSC but not in MC. The existence of different subp opulations of plastid ribosomes-e.g., those with and without L21 and/or L29 during development-evokes interesting, but as yet unanswered, questions ab out the roles of different types of ribosomes in differentiation.