To study the molecular basis of biotrophic nutrient uptake by plant parasitic rust fungi, the gene (Uf-PMA1) encoding the plasma membrane H+-ATPase from Uromyces fabae was isolated. Uf-PMA1 exists probably as a single gene. However, two nearly identical sequences were identified; the similarity apparently is due to two Uf-PMA1 alleles in the dikaryotic hyphae. Multiple Uf-PMA1 transcripts were observed during early rust development, and reduced amounts of a single Uf-PMA1 mRNA were observed in haustoria and rust-infected leaves. This is in contrast to elevated enzyme activity in haustoria compared to germinated spores (C. Struck, M. Hahn, and K. Mendgen. Fungal Genet. Biol. 20:30--35, 1996). Unexpectedly, the PMA1-encoded rust protein is more similar to H+-ATPases from plants (55% identity) than from ascomycetous fungi (36% identity). When the rust PMA1 cDNA was expressed in Saccharomyces cerevisiae, both the wild-type enzyme and a mutant derivative (Δ76) deleted for the 76 C-terminal amino acids were able to support growth of a yeast strain lacking its own H+-ATPases. Compared to the wild-type, the Δ76 mutant enzyme displayed increased affinity to ATP, a higher vanadate sensitivity, and a more alkaline pH optimum. These results indicate that the C-terminal region of the rust enzyme exhibits auto-regulatory properties.