Fig. (1) The HaloTag expression vectors used for protein expression and functional studies Various expression vector systems were used for HaloTag recombinant protein expression. The vectors, pFN18A, pFN19A, pFC20A and pFC14A were obtained from Promega for expression of target proteins in E. coli, cell-free and mammalian expression systems. In order to modify applicable cloning methods, we further modified these vectors to contain the ccdB cassette for positive selection of cloned plasmids. The expression vector, pGW-nHalo is based on pFN18A and ccdB cassette was incorporated into the vector. The pHis-cHalo was based on pFN20A and T02 (pHis) vector . The expression vector, pLIC-Halo was also based on pFC20A and LIC cloning site was incorporated with ccdB cassette. These vectors were used for expression and solubility studies of proteins in S. pneumoniae TIGR4, Y. pestis KIM 10, B. mallei ATCC 23344 and H1N, and for functional analysis.