Scheme of the key components of the purine degradation pathway. The image shows the reversible (double headed arrow) and irreversible (single headed arrow) reactions catalyzed by the corresponding enzymes (rectangular square) for purine degradation. The colors represent up regulation (red) and down regulation (blue) of genes in cells shifted to medium containing adenine as the sole nitrogen source compared to cells grown with nitrate. Black marked symbols indicate intermediates occurring several times in the pathway. Fold change (FC) values of gene expression are given within the colored boxes.