Catalyzes the hydrolytic deamination of guanine, producing xanthine and ammonia. guanine + H(+) + H2O = NH4(+) + xanthine Purine metabolism; guanine degradation; xanthine from guanine: step 1/1. Expressed only during limited or partially limited nitrogen conditions. Can be induced to high levels in the presence of purines or intermediates of the purine catabolic pathway. Expression seems indirectly controlled by TnrA and GlnR. Belongs to the cytidine and deoxycytidylate deaminase family.