In contrast to bacterial, fungal and vertebrate ornithine transcarbamylases (OTCs; EC 22.214.171.124), very little is known about the enzyme in plants. We report here the isolation of a T-DNA-tagged mutant displaying sensitivity to ornithine, whose characterization has allowed for the identification of several complementary and genomic DNA clones encoding the OTC and auxilin-like 1 (AUL1) proteins of the crucifer Arabidopsis thaliana. Transcript mapping revealed that at least 22 bp within the OTC-AUL1 intercoding region are transcribed from both strands, which makes this one of the rarely described cases of convergent and overlapping transcription units in the nuclear genome of a multicellular eukaryote. Transcription of the OTC gene was shown to be ubiquitous in aerial organs of adult plants, whereas that of AUL1 was obscured by the existence of a putative second copy of the gene. The OTC-AUL1 locus maps at the bottom of chromosome 1.