Pycsar (pyrimidine cyclase system for antiphage resistance) provides immunity against bacteriophage. The pyrimidine cyclase (PycC) synthesizes cyclic nucleotides in response to infection; these serve as specific second messenger signals. The signal activates the adjacent effector, leading to bacterial cell death and abortive phage infection. A clade B Pycsar system. The pyrimidine cyclase gene of a two-gene Pycsar system, generates cyclic UMP (cUMP) from UTP probably in response to bacteriophage infection (Probable). Expression of this and adjacent effector XpPycTIR (AC P0DV29) confers resistance to bacteriophage T7. When cells expressing the Pycsar system are infected phage T7 at low multiplicity of infection (0.2 MOI) the culture survives, at 2.0 MOI bacteria enter growth arrest. The same cells enter growth arrest after exposure to 2.5 mM cUMP but not cCMP; the effector protein responds only to the cUMP produced by its cognate NTP cyclase (PubMed:34644530). UTP = 3',5'-cyclic UMP + diphosphate Homodimer. Expression is probably induced by infection with phage T7. Belongs to the adenylyl cyclase class-4/guanylyl cyclase family. Pyrimidine cyclase subfamily.