The action of exogenous gibberellic acid on polysome formation and translation of mRNA in germinating castor-bean seeds.

The amount of protein synthesis in germinating castor-bean seeds has been estimated by the quantitative and qualitative exmainatin of polysomes from the seeds in the presence and absence of gibberellic acid (GA3). Careful optimisation of polysome extraction procedures was required to minimise the ribonuclease activity in the extracts. Ribonuclease activity in seed extracts increased fourfold over the first 5 d of germination. Gibberellic acid stimulated polysome formation about twofold during the first 4 d of germination. It also stimulated the amount of mRNA associated with polysomes by about twofold during the first 3 d of germination. Between days 1 and 5 of germination, polysome formation was primarily limited by mRNA availability. During the period 0-24 h, polysome formation was independent of mRNA levles. The increase in enzyme activities stimulated by GA3 was probably the result of an increase in the amount of cellular mRNA. No evidence was obtained for an action of GA3 on translation other than on the increased production of RNA. Examination of the recruitment of isocitrate-lyase mRNA into polysomes showed that GA3 did not specifically stimulate production of this enzyme.