THE USTILAGO-MAYDIS-NAR1 GENE ENCODING NITRATE REDUCTASE-ACTIVITY - SEQUENCE AND TRANSCRIPTIONAL REGULATION

The nar1 gene was cloned from Ustilago maydis and the 908-amino-acid ( aa) sequence of the encoded protein found to have strong identities wi th other nitrate reductases from fungi and plants. This was especially so in three domains which define enzyme cofactor-binding sites. The g ene was isolated alone and in association with the nir1 gene, suggesti ng that the two genes are closely linked on the chromosome. The phenot ype of a strain in which nar1 had been disrupted was consistent with t he only role of nar1 being in nitrate reduction. Nitrate ions induced a 90-fold increase in nar1 transcript levels, while ammonium ions repr essed transcript levels.