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 effector protein of a two-gene Pycsar system. Upon activation by cyclic UMP (cUMP) degrades cellular NAD(+). Expression of this and adjacent uridylate cyclase BcPycC (AC A0A0J5ZXG5) probably confers resistance to bacteriophage. The genes are probably only expressed in response to bacteriophage infection. This protein probably only responds to cUMP (produced by its cognate NTP cyclase). H2O + NAD(+) = ADP-D-ribose + H(+) + nicotinamide Activated by cyclic UMP (cUMP) and to a lesser extent by cCMP. Purified protein forms large 2-dimensional sheets when incubated with cUMP and shorter filaments in the presence of cCMP. Has an N-terminal cyclic nucleotide-binding domain and a C-terminal Toll/interleukin-1 receptor-like domain (TIR) that probably has the NAD(+) hydrolase activity.