Gene families involving in the ethylene biosynthesis in clementine (Citrus clementina L.): II. 1-amino-cyclopropane-1-carboxylic acid synthase (ACS)

Abstract

1-aminocyclopropane-1-carboxylate synthase (ACS) catalyzes the biosynthetic reaction of 1- aminocyclopropane-1-carboxylate (ACC), a precursor of ethylene. This biosynthetic phase plays an important role in ethylene biosynthesis in higher plants. ACS is the key enzyme regulating the speed of ethylene biosynthetic process. By using the in silico approach, we identified a total of seven genes encoding ACS enzymes in the whole genome of clementine trees. All of the predicted ACS proteins contain from 442 to 547 amino acids and include an Aminotransferase class I/classII conserved domain: Aminotran_1_2 (PF00155). These proteins have high similarity to the ACS of other plants, over 70% when compared to the closest orthologs. They also contain three introns which alternately locate between exonic regions. All of CclACSs share seven conserved domains similar to previously reported ACSs. They as well as also cointain the typical amino acids which play an important role as an enzymatic reaction center. The secondary structure of CclACS has two characterized regions, the \(\alpha\beta\alpha\) sandwich region in the central and smaller αβ region at both Nterminal and C-terminal. All of CclACS proteins share 17 \(\alpha\) helices and 12 \(\beta\) sheets. Based on the phylogenetic tree analysis, seven CclACSs were classified into four distinct groups (A1, A2, A3 and B). Different from other dicotyledon plants, no gene duplication event was reported in clementine after speciation point. Only three out of seven ACS genes expressed in experimental conditions. Two group B genes (CclACS3 and CclACS7, respectively) strongly expressed in the mixture of vegetative tissues, whereas CclACS4 only expressed in productive tissues.