Involved in the biosynthesis of syringomycin E, a cyclic lipodepsinonapeptide toxin with phytotoxic activity (PubMed:9830033). Specifically adenylates L-threonine and loads it onto its peptidyl carrier domain, via a thioester linkage to the phosphopanthetheine moiety (PubMed:9830033, PubMed:16002467). Is highly specific for L-threonine (PubMed:16002467). ATP + holo-[peptidyl-carrier protein] + L-threonine = AMP + diphosphate + L-threonyl-[peptidyl-carrier protein] kcat is 29.1 min(-1) with L-threonine as substrate. kcat is 1.23 min(-1) with L-serine as substrate. Mutant is defective in syringomycin production but produces wild-type levels of syringopeptin. Mutation significantly reduces the virulence of strain B301D in immature sweet cherry fruits. Belongs to the ATP-dependent AMP-binding enzyme family.