Involved in the purine-salvage pathway (PubMed:35338694). Catalyzes the NADPH-dependent conversion of GMP to IMP (PubMed:35338694). Is not essential for viability, but may contribute to the regulation of the purine nucleotide pool by recycling GMP to IMP (PubMed:35338694). IMP + NADP(+) + NH4(+) = GMP + 2 H(+) + NADPH Activity is highest with Rb(+), followed by K(+), NH4(+) and Cs(+). Activity is allosterically regulated by the ATP/GTP ratio in a pH-dependent manner (PubMed:35338694). At pH 7.8, GTP has only a minor positive effect and ATP only a minor negative effect on the activity, however, at lower pH values, the effects of ATP and GTP increase (PubMed:35338694). ATP-dependent inhibition can be restored by increasing GTP concentration (PubMed:35338694). IMP and XMP are competitive inhibitors (PubMed:35338694). Optimum pH is 7.4-7.8. Purine metabolism; IMP biosynthesis via salvage pathway. Homooctamer composed of two tetramers (PubMed:35338694). The oligomerization state is regulated by ligands and pH (PubMed:35338694). Knocked out mutant does not require any purine supplement for the growth and does not exhibit any growth defects in media containing hypoxanthine, guanine and adenine. Belongs to the IMPDH/GMPR family. GuaB1 subfamily.