Nonself recognition through intermolecular disulfide bond formation of ribonucleotide reductase in Neurospora

Abstract

Type I ribonucleotide reductases (RNRs) are conserved across diverse taxa and are essential for the conversion of RNA into DNA precursors. In Neurospora crassa, the large subunit of RNR (UN-24) is unusual in that it also has a nonself recognition function,whereby coexpression of Oak Ridge (OR) and Panama (PA) alleles of un-24 in the same cell leads to growth inhibition and cell death.We show that coexpressing these incompatible alleles of un-24 in N. crassa results in a high molecular weight UN-24 protein complex.A 63-amino-acid portion of the C terminus was sufficient for un-24PA incompatibility activity. Redox active cysteines that are conservedin type I RNRs and essential for their catalytic function were found to be required for incompatibility activity of both UN-24OR and UN-24PA. Our results suggest a plausible model of un-24 incompatibility activity in which the formation of a complex between theincompatible RNR proteins is potentiated by intermolecular disulfide bond formation. © 2013 by the Genetics Society of America.