Involved in the biosynthesis of the antibiotic phosphinothricin tripeptide (PTT), which has bactericidal, fungicidal and herbicidal properties (PubMed:16251301). Adenylates L-alanine and loads it onto a peptidyl carrier domain via a thioester linkage to the phosphopanthetheine moiety (PubMed:16251301). Shows weaker activity with aminobutyric acid and L-serine (PubMed:16251301). ATP + holo-[peptidyl-carrier protein] + L-alanine = AMP + diphosphate + L-alanyl-[peptidyl-carrier protein] Antibiotic biosynthesis. Modular protein that contains a first peptidyl carrier protein domain which bears a phosphopantetheinyl arm to attach the activated amino acid, a condensation domain, an adenylation domain which activates the alanine residue into an aminoacyl-AMP ester and a second peptidyl carrier protein domain. Mutant cannot produce PTT. PhsB cannot take on the function of PhsC and vice versa. Belongs to the NRP synthetase family.