The manganese (II) ion dependent transcriptome of the citric acid overproducer fungus Aspergillus niger

Citric acid is a flagship bulk product of the fermentation industry, produced by the filamentous fungus Aspergillus niger. The biochemistry of citric acid accumulation in A. niger has extensively been studied, but many pieces of the puzzle remain to be elucidated. One is the absolute necessity of adjusting correct levels of Mn (II) ions in the growth medium. Citrate production will only occur when Mn (II) ions are present in very low (< 2 ppb) concentrations. Mn (II) deficiency has multiple effects on A. niger physiology. Mycelia grown under Mn (II)-deficient conditions form pellets, while increasing Mn (II) concentration to 14 ppb or above switches morphology to a filamentous one, accompanied by a rapid decline in citrate production. Mn (II) ions also block citrate uptake, impair protein and DNA synthesis, inhibit triglyceride and phospholipid synthesis. Our hypothesis was that multiple genetic elements are involved in the diverse phenotype that Mn (II) deficiency exerts. To test this, in collaboration with our Canadian partners at Concordia University (Quebec, Montreal), we analyse the transcriptome of the high citrate producer, genome-sequenced A. niger strain (ATCC 2270) by deep RNA sequencing from cultures that are cultivated at three, carefully set Mn (II) concentrations. These data will then be used to identify genes which display an at least 2-fold up- or downregulation in dependence on the availability of Mn (II). They will then be identified by BLASTP against the annotated proteins of A. niger. In short, our proposal aims to investigate the genes that primarily responds to Mn (II) ions in A. niger grown under quasi-industrial settings. The results may contribute to the better understanding and the practical control of one of the most important industrial fermentations.