Relative importance of elements within the SL3-3 virus enhancer for T-cell specificity.

Elements within the enhancer of T-lymphomagenic SL3-3 virus were examined for their contributions to transcriptional activity in T lymphocytes and non-T cells. A region containing two sequences homologous to the enhancer core consensus sequence and a sequence homologous to the binding site for factor LVb was found to have the largest effect on activity. Evidence was obtained that suggests that the activity of this region was greater in T lymphocytes than in non-T cells and that multiple elements within it were necessary for activity. A second region, containing sequences homologous to the binding site of factor NF-I and the glucocorticoid response element, had about a twofold effect on transcription in both T lymphocytes and non-T cell lines. The twofold effect was seen whether the region containing the cores and LVb site was present or not. These results indicate that the most important region for the specificity of SL3-3 enhancer activity and, presumably, for viral leukemogenicity comprises the core elements and the LVb site. DNA-protein-binding studies demonstrated that one cellular factor, S/A-CBF, bound to both core elements, while a second cellular factor, S-CBF, bound to only one of them. In combination with earlier studies, this indicates that cells contain multiple factors that bind to the critical region.