Research Project
Multiple arginases encoded by the complex aga locus are found in N. crassa. Two differentially transcribed mRNAs, 1.4 kb and 1.7 kb, produce 36-kDa and 41-kDa proteins, respectively. The larger transcript and protein are made only in the presence of exogenous arginine. In order to understand the function of these two proteins, strains were constructed that make only one form, TM1, makes the 36-kDa form and TM3, the 41-kDa protein. Interestingly, the ability to utilize arginine as a sole nitrogen source is impaired in these strains. Nomarski microscopy revealed a hyphal branching defect for the two mutant strains. TM1 appears to have normal apical extension, but branching aborts after a short extension of the branch. TM3 which makes only the 41-kDa protein, has difficulty extending hyphae and has a more pronounced branching defect-only branch buds are seen in this strain. Enzyme inhibition and kinetic studies have been performed on crude extracts from these strains. It appears that the two forms share some features, their affinity for arginine and the mode of inhibition by ornithine. However, the two arginases differ in concentration of ornithine for 50% inhibition and the amount of manganese needed for 50% activity. These vegetative growth defects and catalytic properties support a model where the two forms of arginase work together for maximal activity.