Catalyzes the dephosphorylation of inositol 1-phosphate (I-1-P) to yield free myo-inositol, a key metabolite in mycobacteria. a myo-inositol phosphate + H2O = myo-inositol + phosphate Polyol metabolism; myo-inositol biosynthesis; myo-inositol from D-glucose 6-phosphate: step 2/2. When comparing gene expression levels of the four IMPase family genes in exponential cultures of M.tuberculosis, the level of cysQ is the highest, almost equal to sigA; impA and impC are expressed at approximately 40% of this level, while suhB is lowest, at 12% of the cysQ level. Strains lacking this gene show no difference in colony morphology and no differences in levels of phosphatidylinosotol mannosides (PIMs), lipomannan (LM), lipoarabinomannan (LAM) or mycothiol (in the absence of exogenous inositol). Belongs to the inositol monophosphatase superfamily.