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 signals activate the adjacent effector, leading to bacterial cell death and abortive phage infection. A clade D Pycsar system. The pyrimidine cyclase gene of a two-gene Pycsar system, generates cyclic UMP (cUMP) from UTP as well as cGMP from GTP to a lesser extent, has little to no activity on ATP or CTP (PubMed:34644530). Expression of this and adjacent effector MePycTM (AC A0A1C5G2D0) probably confers resistance to bacteriophage. The genes are probably only expressed in response to bacteriophage infection (Probable). GTP = 3',5'-cyclic GMP + diphosphate UTP = 3',5'-cyclic UMP + diphosphate Homodimer. Belongs to the adenylyl cyclase class-4/guanylyl cyclase family. Pyrimidine cyclase subfamily.