Produces N-acetylneuraminate-9-phosphate (Neu5Ac-9-P) from N-acetylmannosamine 6-phosphate. Has no detectable activity towards N-acetylmannosamine or mannose 6-phosphate. an N-acyl-D-mannosamine 6-phosphate + H2O + phosphoenolpyruvate = an N-acylneuraminate 9-phosphate + phosphate Ubiquitous.