A gene encoding a predicted nucleic acid helicase was identified in Arabidopsis thaliana by activation of a promoter trap. An in vivo transcriptional fusion between the helicase gene and a promoterless uidA (gusA) gene was generated, which was expressed specifically in the tapetum and vascular tissues. The tagged gene, designated HVT1 (Helicase in Vascular tissue and Tapetum), encodes a native transcript of approximately 4.4 kb, which is of very low abundance. The predicted HVT1 protein, of 1291 amino acid residues, is homologous to the Drosophila MALELESS, human RNA helicase A and bovine nuclear DNA helicase proteins, and represents the first identified member of a new subgroup within the mle group of the DEAH family. Low stringency genomic Southern blot analysis indicates that at least two structurally related genes exist in Arabidopsis. We suggest that the HVT1 protein may play a role in nucleic acid unwinding in restricted cell types during both vegetative and reproductive phases of the Arabidopsis life cycle.