Molecular physiology of copper in Drosophila melanogaster

A circular RNA-gawky-chromatin regulatory axis modulates stress-induced transcription

In response to heavy metal stress, the RNA-binding protein (RBP) gawky translocates into the nucleus and acts as a chromatin-interacting factor to activate the transcription of many stress-responsive genes. However, the upstream regulators of gawky-mediated transcription and their mechanistic details remain unknown. Here, we identified a class of metal-responsive element-containing circRNAs (MRE circRNAs) which specifically interact with gawky during copper stress. Using classic stress-responsive genes as a readout (Drosophila MT), we found that overexpression of MRE circRNAs led to a significant repression in stress-induced transcription. Mechanistically, MRE circRNAs promote the dissociation of gawky from chromatin and increase its aberrant cytoplasmic accumulation, which ultimately impedes the loading of RNA polymerase II to the active gene loci. The MRE motif serves as an important RNA regulon for maintaining the circRNA-gawky interaction, loss of which impaired the inhibitory effects of MRE circRNAs on gawky. Through RNA-seq analyses, we then identified over 500 additional stress-responsive genes whose induced transcription was attenuated upon MRE circRNA overexpression. Finally, we uncovered the physiological relevance of MRE circRNA-mediated regulation in cellular defense against copper overloading. Taken together, this study proposes that the circRNA-RBP-chromatin axis may represent a fundamental regulatory network for gene expression in eukaryotic cells.

The DEAD-box helicase Hlc regulates basal transcription and chromatin opening of stress-responsive genes

Stress-responsive genes are lowly transcribed under normal conditions and robustly induced in response to stress. The significant difference between basal and induced transcription indicates that the general transcriptional machinery requires a mechanism to distinguish each transcription state. However, what factors specifically function in basal transcription remains poorly understood. Using a classic model stress-responsive gene (Drosophila MtnA), we found that knockdown of the DEAD-box helicase Hlc resulted in a significant transcription attenuation of MtnA under normal, but not stressed, conditions. Mechanistically, Hlc directly binds to the MtnA locus to maintain the accessibility of chromatin near the transcriptional start site, which allows the recruitment of RNA polymerase II and subsequent MtnA transcription. Using RNA-seq, we then identified plenty of additional stress-responsive genes whose basal transcription was reduced upon knockdown of Hlc. Taken together, these data suggest that Hlc-mediated basal transcription regulation is an essential and widespread mechanism for precise control of stress-responsive genes.