Biosynthesis of heme in Escherichia coli is under strict regulatory control since free heme or intermediates of its biosynthesis are potentially toxic for the cell. Under normal physiological conditions a bacterial cell does not have significant levels of free heme. Recombinant hemeproteins with affinity for heme lower than that of intrinsic cell proteins are often only isolated as apo-proteins. Moreover, for a number of hemeproteins expressed as apo-protein in E. coli it is not possible to reconstitute holo-protein in vitro. To circumvent these issues, fully active recombinant hemeproteins are usually expressed with expensive 5-aminolevulinic acid supplementation. In the present work, we construct the helper plasmid pHg expressing glutamyl-tRNA reductase (hemA) a key enzyme catalyzing the rate-limiting reaction in heme biosynthesis in E coli, to avoid the necessity of 5-aminolevulinic acid supplementation. Overexpression of HemA restores the proper balance between protein and heme synthesis so that the newly synthesized recombinant apo-protein is continuously converted to holo-protein. The pHg plasmid is capable Of Supporting high-level expression of microsomal CYP1A1, CYP1A2, CYP21, CYP17s, and mitochondrial CYP11A1. This new expression system provides a simple approach to obtain significant quantities of the active holo-form of recombinant hemeproteins. (c) 2005 Elsevier Inc. All rights reserved.