β-Branches in myxovirescin A1. (A) Structure of myxovirescin A1, with C12 methoxymethyl and C16 ethyl β-branches highlighted in yellow. (Bb) Several proteins are thought to be involved in the introduction of the C12 and C16 β-branches. An acyltransferase (AT), ketosynthase (KS), HMG-CoA synthase homolog (HCS), two enoyl-CoA hydratase homologs (ECH), and free-standing donor and assembly-line acceptor thiolation domains (T) are required for each β-branch incorporation; there are two copies of the donor and acceptor thiolation domains, acyltransferase, and HMG-CoA synthase homolog encoded by the myxovirescin biosynthetic cluster. (C) Schematic of the β-methylation pathway as characterized in bacillaene [2].