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 E Pycsar system. The pyrimidine cyclase gene of a two-gene Pycsar system, weakly generates cyclic CMP (cCMP) from CTP, has little to no activity on ATP, GTP or UTP (PubMed:34644530). Expression of this and adjacent effector SaPycTM (AC P0DV39) probably confers resistance to bacteriophage. The genes are probably only expressed in response to bacteriophage infection (Probable). CTP = 3',5'-cyclic CMP + diphosphate Homodimer. Has an N-terminal nucleotide cyclase domain and a C-terminal nucleotide sensor domain (AGS-C). Belongs to the adenylyl cyclase class-4/guanylyl cyclase family. Pyrimidine cyclase subfamily.