Functional redundancy of octamer elements in the mouse mammary tumor virus promoter.

The promoter of mouse mammary tumor virus contains three overlapping sequence elements related to the octamer consensus (ATGCAAAT) that are largely contained within two 10 bp direct repeats (CTTATGTAAA) separated by a 2 bp spacer between 60 and 39 relative to the start of transcription. Gel electrophoresis mobility shift competition assays demonstrate that the most distal of these octamer-related elements is recognized by a protein that also binds to the most proximal element, while the central octamer-related element is not efficiently recognized. Transient transfection assays with altered promoters reveal that the portion of the 10 bp repeat that is not related to the octamer consensus appears not to be important for transcription. The distal and proximal octamer-related elements are, at least to some extent, functionally redundant. Complete deletion of one element has little or no effect on promoter activity so long as certain spacing constraints among remaining promoter elements are maintained. Systematic variation of such spacing reveals a cyclic effect on promoter activity corresponding to the periodicity of Bform DNA, suggesting functional interactions between proteins bound to adjacent sites.