The transcriptional organization of the gene cluster encoding the F1845 fimbrial adhesin of a diarrhoea-associated Escherichia coli was investigated. Genes daaA to daaE were determined to constitute a single transcriptional unit under the control of the daaA promoter. The nucleotide sequence of daaA and that of an upstream open reading frame encoded on the opposite strand, designated daaF, were determined to share limited homology with the papB and papI genes of the P fimbrial adhesin, respectively. The 5' termini of the daaF and daaABCDE transcripts were mapped by primer extension and nuclease protection analyses. The promoters for these transcripts were associated with potential regulatory sequences including two consensus leucine-responsive regulatory protein (Lrp)-binding sites which contained differentially methylated GATC sequences, a cAMP-CRP-binding site, and an integration host factor (IHF)-binding site. Expression of the daa locus was determined to be dependent on Lrp, subject to catabolite repression, and dependent on IHF.