The site I 22 kDa auxin-binding proteins from maize are encoded by a small gene family comprising at least five members. Here the cloning and molecular analysis of the Zm-ERabp1, Zm-ERabp4, and Zm-ERabp5 genes is presented. All three encode 22-23 kDa proteins displaying a transit peptide, a C-terminal KDEL sequence, as well as glycosylation and auxin-binding sites. The Zm-ERabp4 and Zm-ERabp5 genes are very similar. The Zm-ERabp1 gene encodes a related protein, but its promoter, leader and signal peptide are very different. Northern analysis using gene-specific oligonucleotide probes indicates that Zm-ERabp4 is expressed in leaves and coleoptiles but weakly in roots, whereas Zm-ERabp5 expression is barely detectable in these tissues. RNA-PCR indicated that all three genes are none the less expressed in many tissues. Primer-extension analysis revealed an unusually long (320 bases) Zm-ERabp1 leader containing an 80 codon ORF which, if expressed, would encode a positively charged protein with some similarity to transcription factors. In a transient promoter-reporter gene expression system using maize leaf protoplasts the Zm-ERabp1 promoter is more active than the Zm-ERabp4 and Zm-ERabp5 promoters. Promoter deletion analysis of Zm-ERabp1 has identified a negative regulatory sequence in a region from -364 bp and -130 bp, deletion of which results in about twofold higher expression. This region contains both enhancer- and G-box-related sequences. Deletion of -126 bp to +64 bp, which contains the TATA box and transcription start, results in a large decrease in expression.