Transcriptional regulation of PS-IAA4/5 and PS-IAA6 early gene expression by indoleacetic acid and protein synthesis inhibitors in pea (Pisum sativum).

The transcription of two genes, PS-IAA4/5 and PS-IAA6, in pea is induced by indoleacetic acid (IAA) and protein synthesis inhibitors such as cycloheximide (CHX) and anisomycin (ANI). Induction by IAA is rapid, taking 5 and 7.5 minutes for PS-IAA4/5 and PS-IAA6, respectively, and is independent of IAA concentration and whether IAA has a free or esterified carboxyl group (ethyl-IAA). The rate of mRNA accumulation, however, is dependent on hormone concentration, and is greater with IAA than with ethyl-IAA. The turnover rates (t1/2) of the PS-IAA4/5 and PS-IAA6 mRNAs are 60 and 75 minutes, respectively, and are not affected by IAA. CHX or ANI induce the transcription of PS-IAA4/5 and PS-IAA6 more slowly than IAA (5 to 10 minutes for PS-IAA4/5 and 20 minutes for PS-IAA6). While protein synthesis inhibitors stabilize both mRNAs, the rapidity of induction by CHX and ANI cannot be accounted for solely by mRNA stabilization. The relationship between mRNA induction and protein synthesis inhibition does not obey Michaelis-Menten kinetics, but rather is best described by a hyperbolic curve, suggesting the release of transcriptional repression by the inhibition of protein synthesis. RNA expression experiments with transgenic tobacco seedlings or with transfected pea protoplasts using PS-IAA4/5 promoter GUS or CAT fusions reveal that CHX transcriptionally activates PS-IAA4/5 gene expression. Thus, protein synthesis inhibitors have a dual effect on PS-IAA4/5 and PS-IAA6. (1) They stabilize both mRNAs (possibly by a translational arrest-linked process or by preventing the synthesis of a labile nuclease(s)). (2) They activate transcription (possibly by preventing the synthesis or function of a repressor).