The ocs-like elements of the bidirectional mas1'2' promoter of Agrobacterium tumefaciens, mas1' and mas2', were analyzed to elucidate their role in the expression conferred by this promoter. Tetramers of the elements were cloned upstream of the beta-glucuronidase-coding region linked to the 35S promoter deleted at -54. Transient expression assays with tobacco (Nicotiana tabacum) and potato (Solanum tuberosum) protoplasts showed that tetramers of the mas1' element had 3- to 8-fold enhancing activity, respectively. Enhancement obtained by tetramers of the mas2' element was higher, suggesting that this element plays a role in the stronger promoter activity from the 2' side. Three cDNA clones with higher homology to the tobacco transcription factor TGA1a were isolated from a potato root expression library. Overexpression of the proteins encoded by these cDNA clones in Escherichia coli and analysis of DNA-binding activity in bacterial extracts showed that all three factors could bind strongly to the mas1' ocs-like element. In contrast, only two of the mas-binding factors exhibited significant binding to the mas2' element. Southern analysis revealed the presence of a small, multigene family encoding the mas-binding factors in potato.