Summary

megaterium has served as a model system for the systematic evaluation of single and multiple directed molecular and genetic optimization strategies. Plasmid and genome-based overexpression of genes involved in vitamin B12 biosynthesis, including cbiX, sirA, modified hemA, the operons hemAXCDBL and cbiXJCDETLFGAcysGAcbiYbtuR, and the regulatory gene fnr, significantly increased cobalamin pro-duction. To reduce flux along the heme branch of the tetrapyrrole pathway, an antisense RNA strategy involving silencing of the hemZ gene encoding coproporphyrinogen III oxidase was successfully employed. Feedback inhibition of the initial enzyme of the tetrapyrrole biosynthesis, HemA, by heme was overcome by stabilized enzyme overproduction. Simi-larly, the removal of the B12 riboswitch upstream of the cbiXJCDETLFGAcysGAcbiYbtuR operon and the recombinant production of three different vitamin B12 binding proteins (glutamate mutase GlmS, ribonucle-otide triphosphate reductase RtpR and methionine synthase MetH) partly abolished B12-dependent feedback inhibition. All these strategies increased cobalamin production in B. megaterium. Finally, com-binations of these strategies enhanced the overall intracellular vitamin B12 concentrations but also reduced the volumetric cellular amounts by placing the organism under metabolic stress