A gene encoding 1-aminocyclopropane-1-carboxylate (ACC) synthase produces two transcripts: Elucidation of a conserved response

Abstract

Indole-3-acetic acid (IAA) promotes ethylene biosynthesis in stems of etiolated pea (Pisum sativum L.) seedlings by rapidly increasing the expression of 1-aminocyclopropane-1-carboxylate (ACC) synthase mRNA and by enhancing the activity of the enzyme. Two cDNA clones encoding ACC synthase, Ps-ACS1 and Ps-ACS2, were isolated from a cDNA library prepared from the apical hooks of etiolated pea seedlings that had been treated with 100 μM IAA for 4 h. While studying the expression pattern of IAA-induced ACC synthase mRNA, we observed that the probe for PsACS1 hybridized to two transcripts of 1.6 and 1.9 kb on RNA gel blots. The shorter transcript accumulated before the longer one did, indicating that it is not a degradation product of the latter. Because a similar observation, namely hybridization of one ACC synthase probe to two transcripts, has also been reported in other species, we investigated the relationship between the 1.6- and 1.9-kb transcripts. DNA gel blot analysis using the entire cDNA as probe and RNA gel blot analysis using the 3'-untranslated region as probe indicated that both transcripts are encoded by the same gene. Oligonucleotide-directed RNase H mapping showed that the transcripts differ in the sequence of their 5'-ends. Using 5'-RACE to obtain the DNA sequence of the shorter transcript, we determined that the 1.6-kb transcript (Ps-ACS1b) begins within the second exon of the 1.9-kb transcript (Ps-ACS1a) and lacks the first 383 bases. Thus, Ps-ACS1b does not encode a full-length ACC synthase protein. Because the Ps-ACS1b sequence is identical to that of Ps-ACS1a, including proper splicing of the second intron, Ps-ACS1b appears to result from the use of an alternative, internal promoter.