Sinorhizobium meliloti β-glucosidase (β-D-glucoside glucohydrolase: EC 3.2.1.21) is an exocellulase and cleavages β-(1-4) glycosidic bonds in poly- and oligosaccharides. It has also high activity toward indican, a precursor to indigo and indirubin used as clothing dyes, cosmetics and antimicrobial agents. Typically, this enzyme induced with IPTG under the control of the corresponding promoter was mainly produced as non-functional aggregates in Escherichia coli XL1-Blue and BL21 (DE3). In this study, expression vectors with a mined constitutive promoter (CEM) from metagenome were employed to increase the soluble expression of S. meliloti β-glucosidase. The resulting efficiency of constitutive expression systems was compared with those of the commonly used inducible systems in terms of the specific activity and solubility ratio. Consequently, recombinant β-glucosidases were mainly expressed in soluble form (82-97%) by the constitutive promoter CEM and thus showed the highest specific activity in the recombinant E. coli XL1-Blue harboring pCEMT.