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Expression of the Avirulence Gene Avr9 of the Fungal Tomato Pathogen Cladosporium fulvum Is Regulated by the Global Nitrogen Response Factor NRF1

Here we describe the role of the Cladosporium fulvum nitrogen response factor 1 (Nrf1) gene in regulation of the expression of avirulence gene Avr9 and virulence on tomato. The Nrf1 gene, which was isolated by a polymerase chain reaction-based strategy, is predicted to encode a protein of 918 amino acid residues. The protein contains a putative zinc finger DNA-binding domain that shares 98% amino acid identity with the zinc finger of the major nitrogen regulatory proteins AREA and NIT2 of Aspergillus nidulans and Neurospora crassa, respectively. Functional equivalence of Nrf1 to areA was demonstrated by complementation of an A. nidulans areA loss-of-function mutant with Nrf1. Nrf1-deficient transformants of C. fulvum obtained by homologous recombination were unable to utilize nitrate and nitrite as a nitrogen source. In contrast to what was observed in the C. fulvum wild-type, the Avr9 gene was no longer induced under nitrogen-starvation conditions in Nrf1-deficient strains. On susceptible tomato plants, the Nrf1-deficient strains were as virulent as wild-type strains of C. fulvum, although the expression of the Avr9 gene was strongly reduced. In addition, Nrf1-deficient strains were still avirulent on tomato plants containing the functional Cf-9 resistance gene, indicating that in planta, apparently sufficient quantities of stable AVR9 elicitor are produced. Our results suggest that the NRF1 protein is a major regulator of the Avr9 gene.