Samenvatting

Heme is a suggested limiting factor in peroxidase production by Aspergillus spp., which are well-known suitable hosts for heterologous protein production. In this study, the role of genes coding for coproporphyrinogen III oxidase (hemF) and ferrochelatase (hemH) was analyzed by means of deletion and overexpression to obtain more insight in fungal heme biosynthesis and regulation. These enzymes represent steps in the heme biosynthetic pathway downstream of the siroheme branch and are suggested to play a role in regulation of the pathway. Based on genome mining, both enzymes deviate in cellular localization and protein domain structure from their Saccharomyces cerevisiae counterparts. The lethal phenotype of deletion of hemF or hemH could be remediated by heme supplementation confirming that Aspergillus niger is capable of hemin uptake. Nevertheless, both gene deletion mutants showed an extremely impaired growth even with hemin supplementation which could be slightly improved by media modifications and the use of hemoglobin as heme source. The hyphae of the mutant strains displayed pinkish coloration and red autofluorescence under UV indicative of cellular porphyrin accumulation. HPLC analysis confirmed accumulation of specific porphyrins, thereby confirming the function of the two proteins in heme biosynthesis. Overexpression of hemH, but not hemF or the aminolevulinic acid synthase encoding hemA, modestly increased the cellular heme content, which was apparently insufficient to increase activity of endogenous peroxidase and cytochrome P450 enzyme activities. Overexpression of all three genes increased the cellular accumulation of porphyrin intermediates suggesting regulatory mechanisms operating in the final steps of the fungal heme biosynthesis pathway.