Ribosomes in cotyledons of mungbean seeds at different stages of germination

Abstract

Ribosomes from cotyledons at different stages of germination were prepared and subjected to gradient centrifugation. Bands corresponded to 80, 60, and 40S. By removing and replacing Mg²⁺, the 80S ribosomes dissociated to 60 and 40S and reassociated. The older the cotyledons, the more readily were the ribosomes dissociated. RNA isolated from these ribosomes showed three components, i.e., 25, 16, and 4S. The effect of time of soaking on RNA synthesis was examined; on sucrose gradients, the radioactivity tended to be heterodisperse with a large fraction in the low A₂₆₀ region between 4 and 16S. This region was collected from cotyledons soaked for 6 hr and examined in hybridization experiments with DNA from leaves, adding RNA from 6,-24- and 48-hr cotyledons to investigate competition. With the passage of time the ability to compete was lost, i.e., 6-hr RNA reduced binding by 80% but 48-hr RNA only by 10%, suggesting that a burst of many RNA species occurs on soaking followed by a phasing-out of various species with time. The half-life of ribosomal RNA was found to be 42-45 hr in leaf and root tissue, but no turnover was observed in cotyledons between hour 24 and 48. Preliminary studies on the ribosomal protein of cotyledons using gel electrophoresis technique, indicate that two slow-moving protein bands are present whose density decreases with time of soaking, but these are not present in root ribosomes.