Phytochrome-mediated Assembly of Polyribosomes in Etiolated Bean Leaves. Evidence for Post-transciptional Regulation of Development

Light operating through phytochrome controls the proportion of total ribosomes present as polyribosomes in etiolated leaves of Phaseolus vulgaris. Similar responses but with slightly different time courses are elicited by brief red light treatment and by continuous far-red or white light. The increase in polyribosome proportions after red light treatment reaches a maximum within 2 h, after which the proportion steadily declines. Light treatment appears to lead to increased proportions of polyribosomes in higher size classes. This is most evident with continuous white light, but probably also occurs with red and far-red light. The increase in polyribosomes is due principally to cytoplasmic ribosomes although proportionately greater effects are observed in chloroplast ribosomes. Although cordycepin inhibits RNA synthesis by 85-90% within 3 h there is no depression of the light-mediated increase in polyribosome proportions when leaves are preincubated in the inhibitor for 3 h. The data therefore indicate that phytochrome is capable of controlling protein synthesis, and thus development, at a post-transciptional level.