The "fas" is an operon made of six genes (orf 1-6) and a regulatory gene, "fasR". Because deletions of some "fas" genes give a non-virulent phenotype, for fas a main role in virulence was proposed . Gene "fasR" is an araC-like transcriptional regulator. Its transcription can be induced "in vitro" in cultures containing certain carbon sources (such as glucose, sucrose, arabinose, glycerol, pyruvate, mannitol, mannose) or nitrogen sources (such as histidine), and is influenced by culture pH and optical density. Also, fasR can be induced by gall extract created by virulent strain.