The bean rust fungus, Uromyces appendiculatus, undergoes thigmotropic differentiation to produce infection structures. Six differentiation-specific genes have been isolated and one, INF24, has been characterized [Bhairi et al., Gene 81 (1989) 237-243]. Here, we report the structure of a second gene, INF56, which was subcloned on a 2.6-kb fragment and sequenced. The location of the 1.0-kb INF56 transcript was determined by S1 nuclease protection and primer extension. A TATA box was found 38 bp upstream and a CAAT box 130 bp upstream from the major transcription start point (tsp). The gene contains two open reading frames: ORF2 is nested within ORF1; they share a 67-bp intron. ORF1 encodes a 14.1-kDa polypeptide which has an amino acid sequence rich in Gly, Pro and Ser. It has sequence similarity to a functional domain (V2) of mammalian cytokeratin type II. ORF2 encodes a 10.1-kDa polypeptide which is rich in Pro. It shares similarity with the cell-surface recognition region of chicken fibronectin. Hybrid selection and in vitro translation of the INF56 mRNA yielded two polypeptides of 15.5 and 23 kDa, as estimated by sodium dodecyl sulfate-polyacrylamide-gel electrophoresis. INF56 is constitutively expressed at a low level, but the abundance of its steady-state transcript is upshifted 4.5 h after spore hydration during the period that infection structures are formed.