Chemically competent E. coli cells derived from BL21(DE3). Poly-histidine tagged recombinant proteins that are isolated by immobilized metal affinity chromatography (IMAC) are often contaminated with significant amounts of endogenous E. coli metal binding proteins. The protein expression strain NiCo21(DE3) has been engineered to minimize E. coli protein contamination of IMAC fractions: GlmS is mutated to eliminate binding to IMAC resins and three other proteins (SlyD, ArnA and Can) are tagged to enable rapid removal by chitin affinity chromatography.