We compared the effects of phosphorylation of Oct-2 protein on its binding to the consensus octamer sequence (ATGCAAAT) and two non-canonical sequences present in human (AAGCAAAT) and murine (AAACAAAT) promoters of the BLR1 (Burkitts' lymphoma receptor 1) gene encoding chemokine receptor CXCR5 (CXC-chemokine receptor 5). The latter cis-acting elements represent low-affinity recognition sequences for the octamer transcription factors. Okadaic acid was found to induce hyperphosphorylation of Oct-2 specifically in cells of lymphoid lineage. Potentially phosphorylated amino acid residues localized to the POU-specific domain of Oct-2. Whereas binding of Oct-2 to the octamer site from the human BLR1 promoter or to the consensus octamer sequence was unaffected by phosphorylation of this factor, a strong reduction of Oct-2 binding to the octamer site from the murine BLR1 promoter was observed. This finding correlates well with the down-regulation of expression of the BLR1 gene in murine splenic cells but not in lymphoid cells of human origin treated with okadaic acid. These data support the hypothesis that phosphorylation of Oct-2 may be a mechanism by which activities of the promoters containing non-canonical octamer sequences are differentially regulated in response to extracellular stimuli.

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