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Background  The industrially relevant filamentous fungus  Aspergillus niger  is widely used in industry for its secretion capabilities of enzymes and organic acids. Biotechnologically produced organic acids promise to be an attractive alternative for the chemical industry to replace petrochemicals. Itaconic acid (IA) has been identified as one of the top twelve building block chemicals which have high potential to be produced by biotechnological means. The IA biosynthesis cluster ( cadA, mttA  and  mfsA ) has been elucidated in its natural producer  Aspergillus terreus  and transferred to  A. niger  to enable IA production. Here we report the rewiring of a secondary metabolite pathway towards further improved IA production through the overexpression of a putative cytosolic citrate synthase  citB  in a  A. niger  strain carrying the IA biosynthesis cluster.    Results  We have previously shown that expression of  cadA  from  A. terreus  results in itaconic acid production in  A. niger  AB1.13, albeit at low levels. This low-level production is boosted fivefold by the overexpression of  mttA  and  mfsA  in itaconic acid producing AB1.13 CAD background strains. Controlled batch cultivations with AB1.13 CAD + MFS + MTT strains showed increased production of itaconic acid compared with AB1.13 CAD strain. Moreover, preliminary RNA-Seq analysis of an itaconic acid producing AB1.13 CAD strain has led to the identification of the putative cytosolic citrate synthase  citB  which was induced in an IA producing strain. We have overexpressed  citB  in a AB1.13 CAD + MFS + MTT strain and by doing so hypothesize to have targeted itaconic acid production to the cytosolic compartment. By overexpressing  citB  in AB1.13 CAD + MFS + MTT strains in controlled batch cultivations we have achieved highly increased titers of up to 26.2 g/L IA with a productivity of 0.35 g/L/h while no CA was produced.    Conclusions  Expression of the IA biosynthesis cluster in  Aspergillus niger  AB1.13 strain enables IA production. Moreover, in the AB1.13 CAD strain IA production resulted in overexpression of a putative cytosolic citrate synthase  citB . Upon overexpression of  citB  we have achieved titers of up to 26.2 g/L IA with a productivity of 0.35 g/L/h in controlled batch cultivations. By overexpressing  citB  we have also diminished side product formation and optimized the production pathway towards IA.    Electronic supplementary material  The online version of this article (doi:10.1186/s12934-016-0527-2) contains supplementary material, which is available to authorized users.

Rewiring a secondary metabolite pathway towards itaconic acid production in Aspergillus niger