Part of the bacABCDEF operon responsible for the biosynthesis of the nonribosomally synthesized dipeptide antibiotic bacilysin, composed of L-alanine and L-anticapsin. Bacilysin is an irreversible inactivator of the glutaminase domain of glucosamine synthetase (PubMed:15609023, PubMed:20052993). BacA is an unusual prephenate decarboxylase that avoids the typical aromatization of the cyclohexadienol ring of prephenate (PubMed:20052993)(PubMed:22483065). BacA catalyzes the protonation of prephenate (1-carboxy-4-hydroxy-alpha-oxo-2,5-cyclohexadiene-1-propanoic acid) at C6 position, followed by a decarboxylation to produce the endocyclic-delta(4),delta(8)-7R-dihydro-hydroxyphenylpyruvate (en-H2HPP) (PubMed:20052993, PubMed:22483065, PubMed:22765234). En-H2HPP is able to undergo a slow nonenzymatic isomerization to produce the exocyclic-delta(3),delta(5)-dihydro-hydroxyphenylpyruvate (ex-H2HPP) (PubMed:20052993)(PubMed:22483065). BacA isomerizes only the pro-R double bond in prephenate (PubMed:22483065). H(+) + prephenate = 3-[(4R)-4-hydroxycyclohexa-1,5-dien-1-yl]-2-oxopropanoate + CO2 kcat is 190 min(-1) for decarboxylase activity with prephenate as substrate. Antibiotic biosynthesis; bacilysin biosynthesis. The compound guanosine 5'-diphosphate 3'-diphosphate (ppGpp) is essential for the transcription of the bacABCDEF operon and BacG, and GTP regulates the transcription of both this operon and ywfH via the CodY-mediated regulation system. Belongs to the prephenate decarboxylase family.