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 B Pycsar system. The pyrimidine cyclase gene of a two-gene Pycsar system, generates cyclic UMP (cUMP) from UTP, has little to no activity on ATP, CTP or GTP (PubMed:34644530). Expression of this and adjacent effector BcPycTIR (AC A0A0J5WTU0) probably confers resistance to bacteriophage. The genes are probably only expressed in response to bacteriophage infection (Probable). UTP = 3',5'-cyclic UMP + diphosphate Slightly more active with Mn(2+) than with Mg(2+). Optimum pH is 7.0-9.5. Homodimer. Belongs to the adenylyl cyclase class-4/guanylyl cyclase family. Pyrimidine cyclase subfamily.