Hydroxymethylglutaryl-CoA synthase involved in the biosynthesis of siderophore ferrichrome A which is contributing to organismal virulence (PubMed:20070524). The first step of ferrichrome A biosynthesis is performed by the HMG-CoA synthase hcs1 which catalyzes the generation of HMG-CoA and CoA using acetoacetyl-CoA and acetyl-CoA as substrates (PubMed:20070524). The enoyl-CoA isomerase/hydratase fer4 then catalyzes the conversion of hcs1-produced HMG-CoA to methylglutaconyl-CoA (PubMed:20070524). The acyltransferase fer5 then fuses the fer4-generated methylglutaconyl-CoA with sid1-generated hydroxyornithine to yield methylglutaconyl hydroxyornithine (PubMed:20070524). Methylglutaconyl hydroxyornithine is then available for use by the NRPS fer3 to generate ferrichrome A (PubMed:20070524). acetoacetyl-CoA + acetyl-CoA + H2O = (3S)-hydroxy-3-methylglutaryl-CoA + CoA + H(+) Siderophore biosynthesis. Leads to a swollen morphology and a cytokinesis defect preceding lysis of cells (PubMed:20070524). Belongs to the thiolase-like superfamily. HMG-CoA synthase family.