ASSOCIATION OF PLANT P40 PROTEIN WITH RIBOSOMES IS ENHANCED WHEN POLYRIBOSOMES FORM DURING PERIODS OF ACTIVE TISSUE-GROWTH

p40s are acidic proteins of eukaryotic cells occurring either free in the cytoplasm or in association with ribosomes, the latter occurring i n both monosomes and polysomes. p40s may play a role in the regulation of protein synthesis, although the exact mechanism is not known. Leav es of all 10 plant species examined here, including both monocots and dicots, contained proteins detected on immunoblots with Arabidopsis th aliana p40 antiserum. The number and apparent size of the protein band s were variable even among closely related species. Abundance of p40 r elative to ribosomal content during soybean (Glycine max L.) seed germ ination and during seed and leaf development was examined. p40 abundan ce correlated with periods of active tissue growth and high polysome c ontent. The plant growth regulator indole acetic acid caused an increa se in polysome formation in etiolated pea (Pisum sativum L.) plants an d a concomitant recruitment of p40 into polysomes. Subcellular localiz ation at the microscopy level indicated that the pattern of p40 staini ng is very similar to that for RNA, except that p40 is excluded from t he nucleus. These data suggest that p40 is an accessory protein of the ribosome that might play a role in plant growth and development.