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 induc ed 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 este rified carboxyl group (ethyl-IAA). The rate of mRNA accumulation, howe ver, is dependent on hormone concentration, and is greater with IAA th an with ethyl-IAA. The turnover rates (t(1/2)) 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 m ore 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 b y mRNA stabilization. The relationship between mRNA induction and prot ein 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 fusion s reveal that CHX transcriptionally activates PS-IAA4/5 gene expressio n. Thus, protein synthesis inhibitors have a dual effect on PS-IAA4/5 and PS-IAA6. (1) They stabilize both mRNAs (possibly by a translationa l arrest-linked process or by preventing the synthesis of a labile nuc lease(s)). (2) They activate transcription (possibly by preventing the synthesis or function of a repressor). (C) 1995 Academic Press Limite d