We previously identified a gene, termed Mblk-1, that encodes a putative transcription factor with two DNA-binding motifs expressed preferentially in the mushroom body of the honeybee brain, and its preferred binding sequence, termed Mblk-1-binding element (MBE) (Takeuchi, H., Kage, E., Sawata, M., Kamikouchi, A., Ohashi, K., Ohara, M., Fujiyuki, T., Kunieda, T., Sekimizu, K., Natori, S., and Kubo, T. (2001) Insect Mol Biol 10, 487-494; Park, J.-M., Kunieda. T., Takeuchi, H., and Kubo, T. (2002) Biochem. Biophys. Res. Commun. 291, 23-28). In the present study, the effect of Mblk-1 on transcription of genes containing MBE in Drosophila Schneider's Line 2 cells was examined using a luciferase assay. Mblk-1 expression transactivated promoters containing MBEs approximately 2-7-fold. Deletion experiments revealed that RHF2, the second DNA-binding domain of Mblk-1, was necessary for the transcriptional activity. Furthermore, mitogen-activated protein kinase (MAPK) phosphorylated Mblk-1 at Ser-444 in vitro, and the Mblk-1-induced transactivation was stimulated by phosphorylation of Ser-444 by the Ras/MAPK pathway in the luciferase assay. These results suggest that Mblk-1 is a transcription factor that might function in the mushroom body neuronal circuits downstream of the Ras/MAPK pathway in the honeybee brain.