Non-reducing polyketide synthase; part of the gene cluster that mediates the biosynthesis of atranorin, a depside of polyketide origin that accumulates in the cortical or medullary layers of lichen thalli (PubMed:34154413). The first step in the pathway is performed by the non-reducing polyketide synthase atr1 that produces 4-O-demethylbarbatic acid composed of two 3-methylorsellinic acid (3MOA) moieties from S-adenosyl-L-methionine (SAM), acetyl-CoA and malonyl-CoA units (PubMed:34154413). The pathway continues with the actions of the cytochrome P450 monooygenase atr2 that catalizes the oxidation of c-9 and the O-methyltransferase atr3 that performs the methylation of the carboxyl group to yield atranorin, via the proatranorin II and III intermediates if atr2 acts first, or the proatranorin I intermediate if atr3 acts first (PubMed:34154413). Secondary metabolite biosynthesis; terpenoid biosynthesis. Multidomain protein; including a starter unit:ACP transacylase (SAT) that selects the starter unit; a ketosynthase (KS) that catalyzes repeated decarboxylative condensation to elongate the polyketide backbone; a malonyl-CoA:ACP transacylase (MAT) that selects and transfers the extender unit malonyl-CoA; a product template (PT) domain that controls the immediate cyclization regioselectivity of the reactive polyketide backbone; and an acyl-carrier protein (ACP) that serves as the tether of the growing and completed polyketide via its phosphopantetheinyl arm. The release of the polyketide chain from the non-reducing polyketide synthase is mediated by the thioesterase (TE) domain localized at the C-ter of the protein.