Transformation systems are important for an understanding of gene expression because they provide the link between gene structure determined in vitro and gene function assayed in vivo. Transformation of fungi has been studied and used quite extensively. By using transformation, the nit-2 gene of Neurospora crassa was cloned. The nit-2 gene is known to be the major regulatory gene for "nitrogen catabolite repression" in Neurospora. It has a positive effect on the expression of various nitrogen metabolizing enzymes and permeases. The nit-2 gene was subcloned and regulation of nit-2 gene expression was studied. Different proposed control mechanisms for the operation of the nitrogen control circuit in Neurospora were also examined in order to gain new information and to reveal the relationships between nit-2 gene and proposed control elements of nit-2 (nmr-1 and gln-1).;The results indicate that the nit-2 gene expression is itself subject to enhanced expression during nitrogen limitation and show that nit-2 is not controlled by autogenous regulation. Both the unlinked genes nmr-1('+) and gln-1('+) appear to positively control the transcription of nit-2. Experiments even carried out which strongly imply that the nit-2 gene encodes a regulatory protein.
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