The MtrAB histidine-aspartate signal transduction of mycobacteria includes the response regulator MtrA and sensor kinase MtrB. We recently showed that Mycobacterium smegmatis ΔmtrB is filamentous, defective for cell division, cell shape maintenance and shows compromised MtrA target gene expression. Interestingly, overproduction of phosphorylation competent M. tuberculosis MtrA_Y102C reverses the ΔmtrB mutant phenotype, although the genetic basis of phenotype reversal is unknown. Here we show that introduction of D56N mutation in MtrA_Y102C completely abolished its phosphorylation potential yet the double mutant protein retained a partial ability to reverse the mtrB mutant phenotype indicating that phosphorylation activity is not necessary for the function of MtrA_Y102C. The phosphorylation-defective MtrA_D56N-Y102C protein bound its target promoters ripA and fbpB efficiently. Together, these results support a hypothesis that the gain-of-function phenotype of MtrA_Y102C is in part due to its ability to function as a constitutively active protein in the absence of phosphorylation.