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 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 adjacent effector PaPycTIR (AC P0DV41) probably confers resistance to bacteriophage. The genes are probably only expressed in response to bacteriophage infection (Probable). Does not have adenylyl or guanylyl cyclase activity (PubMed:32454240). UTP = 3',5'-cyclic UMP + diphosphate When crystallized with (non-physiological) ATP it binds both Ca(2+) and Mn(2+) (PubMed:32454240). The enzyme assay showing cUMP synthase activity has Mn(2+) and Mg(2+) (PubMed:34644530). Monomer. The 2 guanylate cyclase domains fold into a pseudo-dimeric structure; the second domain binds metal while the first provides most of the nucleotide-binding residues. Belongs to the adenylyl cyclase class-4/guanylyl cyclase family. Pyrimidine cyclase subfamily.