Involved in the initiation of the fatty acid biosynthesis. Catalyzes the condensation of acetyl coenzyme A (acetyl-CoA) with malonyl-acyl carrier protein (ACP) to make the fatty acid synthesis (FAS) primer beta-acetoacetyl-ACP. It can also use short-chain acyl-CoA as substrates, including butyryl-CoA, and hexanoyl-CoA, but does not use any of the longer chain acyl-CoA substrates. acetyl-CoA + H(+) + malonyl-[ACP] = 3-oxobutanoyl-[ACP] + CO2 + CoA Lipid metabolism; fatty acid biosynthesis. Homodimer. Cells lacking this gene show an attenuated growth due to a defect in de novo fatty acid biosynthesis. Siderophore secretion and quorum-sensing signaling, particularly in the rhl and Pseudomonas quinolone signal (PQS) systems, is significantly muted in the absence of fabY (PubMed:22753059). In vitro, the deletion of fabY results in an increased susceptibility to a number of antibiotics, including vancomycin and cephalosporins. The antibiotic susceptibility is influenced by changes in membrane lipid composition which lack a single secondary lauroyl group, resulting in hypoacylated lipid A (PubMed:24145528). FabY is a FabB/F homolog that functions as a FabH. Belongs to the thiolase-like superfamily. Beta-ketoacyl-ACP synthases family.