Synergistic activation of human and viral promoters upon cotransfection of B-MYB with cyclins. (A) Five micrograms of HIV-1-LTR (long terminal repeat) and cdc2 promoter constructs was cotransfected with 10 μg of B-MYB expression vector with or without 5 μg of CMV-cyclin A or CMV-cyclin E. CAT assays were performed as described in the text. (B) One microgram of pGL2-promoter was cotransfected with 1 μg of CMV-B-MYB with or without 1 μg of CMV-cyclin A or CMV-cyclin E in 35-mm dishes and each experiment measurement was performed in triplicate. Luciferase assay was carried out 36 h after transfections, and luciferase activity was assayed with the aid of a scintillation counter and expressed as thousands of cpm. (C) The CD34 promoter linked to CAT was cotransfected with either CMV-B-MYB or SV40-c-MYB, with or without cyclin A, as described for A. (D) The cdc2 promoter, bearing mutations in the myb-binding sites, linked to CAT was cotransfected with CMV-B-MYB, with or without CMV-cyclin A or E, as described for A. Standard deviations are indicated by the error bars.