Drosophila architectural proteins M1BP and Opbp cooperatively form the active promoter of a ribosomal protein gene.

Background: In Drosophila, architectural proteins are frequently found in promoters, including those of genes with extremely high expression levels, such as ribosomal protein genes (RPGs). The involvement of several of these proteins in gene regulation in Drosophila has been shown, but the exact mechanisms of their possible cooperative action have not been fully elucidated.

Results: In this study we dissected the contribution of the architectural proteins Opbp and M1BP, which are co-localized at several RPG promoters near the transcription start site, to promoter functioning. We found that Opbp has two domains that directly interact with CP190, Putzig (Pzg), and Chromator (Chro) proteins, the cofactors which are required for the activation of housekeeping (hk) gene promoters. These domains have redundant functions in vivo and can tether the cofactors forming open chromatin regions when are artificially recruited to the "closed" chromatin. Additionally, we observed interactions between M1BP and the same cofactors. In the transgene assay, the transcription driven by the 192-bp part of Rpl27A RPG promoter is fully dependent on the presence of at least one Opbp or M1BP binding site and it is sufficient for the very high activity of this promoter integrated into the hk gene cluster and moderate expression outside the cluster, while presence of both sites even more facilitates transcription.

Conclusions: This study demonstrates that different architectural proteins can work independently and in cooperation and fulfill partially redundant functions in the activation of RPG promoters.