Nonribosomal peptide synthetase; part of the gene clusters that mediate the biosynthesis of the host-selective toxins (HSTs) ACT-toxins responsible for brown spot of tangerine disease by the tangerine pathotype which affects tangerines and mandarins (PubMed:19271978). ACT-toxins consist of three moieties, 9,10-epoxy-8-hydroxy-9-methyl-decatrienoic acid (EDA), valine and a polyketide (PubMed:22846083). ACT-toxin I is toxic to both citrus and pear; toxin II the 5''-deoxy derivative of ACT-toxin I, is highly toxic to pear and slightly toxic to citrus (PubMed:22846083). On cellular level, ACT-toxins affect plasma membrane of susceptible cells and cause a sudden increase in loss of K(+) after a few minutes of toxin treatment (PubMed:22846083). The acyl-CoA ligase ACTT1, the hydrolase ACTT2, the enoyl-CoA hydratases ACTT3 and ACTT6, and the acyl-CoA synthetase ACTT5 are all involved in the biosynthesis of the AK-, AF- and ACT-toxin common 9,10-epoxy-8-hydroxy-9-methyl-decatrienoic acid (EDA) structural moiety (PubMed:18944496, PubMed:18986255, PubMed:19271978). The exact role of each enzyme, and of additional enzymes identified within the AF-toxin clusters have still to be determined (PubMed:18944496, PubMed:18986255, PubMed:19271978). On the other hand, ACTTS1 to ACTTS4 are specific to the tangerine pathotype (PubMed:22846083). The function of ACTTS3 is to elongate the polyketide chain portion of ACT-toxin that is unique to this toxin (PubMed:20192828). The enoyl-reductase ACTTS2 might complement the missing enoyl-reductase (ER) domain in ACTTS3 in the synthesis of the polyketide portion of ACT-toxin (PubMed:20055645). The roles of the nonribosomal peptide synthetases-related proteins ACTTS1 and ACTTS4 have also still not been elucidated (PubMed:22846083). Mycotoxin biosynthesis. NRP synthetases are composed of discrete domains (adenylation (A), thiolation (T) or peptidyl carrier protein (PCP) and condensation (C) domains) which when grouped together are referred to as a single module (By similarity). Each module is responsible for the recognition (via the A domain) and incorporation of a single amino acid into the growing peptide product. Thus, an NRP synthetase is generally composed of one or more modules and can terminate in a thioesterase domain (TE) that releases the newly synthesized peptide from the enzyme. Occasionally, methyltransferase domains (responsible for amino acid methylation) are present within the NRP synthetase (By similarity). ACTTS4 has the following architecture: C-A-T-C (Probable). Gene clusters encoding host-selective toxins (HSTs) are localized on conditionally dispensable chromosomes (CDCs), also called supernumerary chromosomes, where they are present in multiple copies (PubMed:18986255). The CDCs are not essential for saprophytic growth but controls host-selective pathogenicity (PubMed:18986255). Although conventional disruption of ACTT2 could not be accomplished due to the high number of the copies identified in the genome, the high sequence identity among these copies of ACTT2 is likely an advantage for RNA silencing, because it allows knockdown of all copies of this gene simultaneously (PubMed:18986255). Belongs to the NRP synthetase family.