The capacity to form H-DNA cannot substitute for GAGA factor binding to a (CT)n•(GA)n regulatory site

Previous studies of the Drosophila melanogaster hsp26 gene promoter have demonstrated the importance of a homopurine.homopyrimidine segment [primarily (CT)(n).(GA)(n)] for chromatin structure formation and gene activation. (CT)(n) regions are known to bind GAGA factor, a dominant enhancer of PEV thought to play a role in generating an accessible chromatin structure. The (CT)(n) region can also form an H-DNA structure in vitro under acidic pH and negative supercoiling; a detailed map of that structure is reported here. To test whether the (CT)(n) sequence can function through H-DNA in vivo, we have analyzed a series of hsp26-lacZ transgenes with altered sequences in this region. The results indicate that a 25 bp mirror repeat within the homopurine.homopyrimidine region, while adequate for H-DNA formation, is neither necessary nor sufficient for positive regulation of hsp26 when GAGA factor-binding sites have been eliminated. The ability to form H-DNA cannot substitute for GAGA factor binding to the (CT)(n) sequence.