A GENE ENCODING 1-AMINOCYCLOPROPANE-1-CARBOXYLATE (ACC) SYNTHASE PRODUCES 2 TRANSCRIPTS - ELUCIDATION OF A CONSERVED RESPONSE

Indole-3-acetic acid (IAA) promotes ethylene biosynthesis in stems of etiolated pea (Pisum sativum L.) seedlings by rapidly increasing the e xpression 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 prep ared from the apical hooks of etiolated pea seedlings that had been tr eated with 100 mu M IAA for 4 h. While studying the expression pattern of IAA-induced ACC synthase mRNA, we observed that the probe for Ps-A CS1 hybridized to two transcripts of 1.6 and 1.9 kb on RNA gel blots. The shorter transcript accumulated before the longer one did, indicati ng that it is not a degradation product of the latter. Because a simil ar 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 usi ng the 3'-untranslated region as probe indicated that both transcripts are encoded by the same gene. Oligonucleotide-directed RNase H mappin g showed that the transcripts differ in the sequence of their 5'-ends. Using 5'-RACE to obtain the DNA sequence of the shorter transcript, w e determined that the 1.6-kb transcript (Ps-ACS1b) begins within the s econd 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 prot ein. Because the Ps-ACS1b sequence is identical to that of Ps-ACS1a, i ncluding proper splicing of the second intron, Ps-ACS1b appears to res ult from the use of an alternative, internal promoter.