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 nearby effector, leading to bacterial cell death and abortive phage infection. A clade A 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 effector RsPycTM (AC A0A4R2UGS4) probably confers resistance to some bacteriophage. The genes are probably only expressed in response to bacteriophage infection (Probable). UTP = 3',5'-cyclic UMP + diphosphate Monomer. The 2 guanylate cyclase domains fold into a pseudo-dimeric structure. Belongs to the adenylyl cyclase class-4/guanylyl cyclase family. Pyrimidine cyclase subfamily.