Enhancement of transcription by short alternating C.G tracts incorporated within a Rous sarcoma virus-based chimeric promoter: in vivo studies.

In view of the wide chromosomal distribution of short alternating purine-pyrimidine sequences capable of adopting a number of superhelical stress-dependent structural configurations (left-handed helices and cruciforms), the question has been posed whether such sequences exert any functional effects in vivo. A series of eukaryotic expression vectors were constructed which contained C.G tracts of various lengths in the promoter region. It was shown that insertion of C.G tracts of 12-16 bp significantly increased the level of expression of the chloramphenicol acetyltransferase reporter gene. It was also demonstrated that the formation of additional activation complexes and the use of a preferred "face" or side of the DNA molecule is not responsible for the increased transcription which was observed upon insertion of the C.G tracts. Comparative assays of chromatin structure at the chimeric promoters indicate that the alternating C.G tracts adopt a structure which is incapable of binding histone proteins. These results strongly suggest that control of access to chromatin is involved in regulating the transcriptional activity of the chimeric promoters. Possible molecular bases for this phenomena are discussed.