Human T cell lymphotropic virus type I (HTLV-I) encodes the trans-activator, Tax, which facilitates viral transcription from three 21 bp repeated elements within the U3 region of the long terminal repeat (LTR). Electrophoretic mobility shift (EMS) analyses utilizing double-stranded (ds) oligonucleotides homologous to each of the 21 bp repeats and nuclear extracts derived from selected cell lines of lymphocytic, neuronal, and glial origin have demonstrated the differential binding of cellular factors to each of the three 21 bp repeats. Specifically, both a glial cell-specific DNA-protein complex (designated GCS) and 21 bp repeat-specific DNA-protein complexes (designated U1 and U2) were detected. The formation of the GCS DNA-protein complex may involve activating transcription factor (ATF)/cAMP-response element (CRE) binding protein (CREB) family member(s) while the formation of the U1 and U2 DNA-protein complexes may involve an Sp1-related factor. In addition, three ATF-CREB-related DNA-protein complexes common to each individual 21 bp repeat (designated C1-C3) were also detected. However, we demonstrated that the abundance of the C1 and C2 DNA-protein complexes detected with the individual 21 bp repeats and glial cell nuclear extract was relatively low compared to that obtained with lymphocyte, monocyte, or neuronal nuclear extracts. We also have demonstrated that the ATF-CREB factors participating formation of the GCS DNA-protein complex are distinct from those participating in formation of the C1-C3 DNA-protein complexes. Based on nucleotide sequence requirements and immunoreactivity, we suggest that the GCS DNA-protein complex may contain a novel glial cell type specific ATF-CREB-related factor(s). Furthermore, we demonstrate that the CRE modulator (CREM) protein in conjunction with the ATF/CREB factor, CREBP1, interact with each of the three 21 bp repeats to form the C3 DNA-protein complex. However, the abundance of the C3 DNA-protein complex formed utilizing the promoter proximal repeat is dramatically lower compared to either of the other two 21 bp repeat elements. Based on these observations, we suggest that the differential binding of cellular factors to each of the three 21 bp repeat elements may play a role in basal as well as Tax-mediated LTR-directed transcription within cell populations of either immune or nervous system origin.