MULTIPLE FORMS OF ARGINASE ARE DIFFERENTIALLY EXPRESSED FROM A SINGLE-LOCUS IN NEUROSPORA-CRASSA

The Neurospora crassa catabolic enzyme, arginase (L-arginine amidinohy drolase, EC 3.5.3.1), exists in multiple forms. Multiple forms of argi nase are found in many vertebrates, but this is the only reported exam ple in a microbial organism. The two major forms are structurally simi lar with subunit sizes of 36 and 41 kDa, respectively. The larger form is produced by mycelia growing in arginines-supplemented medium. Both forms are localized in the cytosol. The structural gene for arginase, aga, has been cloned and sequenced; it contains a 358-codon open read ing frame with three in-frame ATGs at the amino terminus. Mutagenesis of these ATGs revealed that the first ATG initiates the 41-kDa protein and the third ATG initiates the 36-kDa protein. Mutation of the secon d ATG has no effect on translation. Northern analysis demonstrated tha t a 1.4-kilobase (kb) transcript is synthesized in minimal medium and both a 1.4- and 1.7-kb transcript are produced in arginine-supplemente d medium. Primer extension identified the 5' ends of each transcript a nd demonstrated that the first and third ATG of the open reading frame are the initial AUGs of the 1.7- and 1.4-kb mRNA, respectively. The r esults suggest that a basal promoter produces the 1.4-kb transcript an d an arginine ''activated'' promoter is responsible for the 1.7-kb tra nscript. Tandem promoters are rare in eukaryotic organisms, and they o ften regulate developmental or tissue-specific gene expression. The po ssibility that arginase has a role in differentiation in N. crassa is being investigated.