A single high-zinc activation enhancer can control two genes oriented head-to-head in C. elegans
Enhancers play critical roles in gene expression, but a full understanding of their complex functions has yet to be defined. The cellular response to excess zinc levels in C. elegans requires the HIZR-1 transcription factor, which binds the high-zinc activation (HZA) enhancer in the promoters of multiple target genes. Cadmium hijacks the excess zinc response by binding and activating HIZR-1. By analyzing the genome-wide transcriptional response to excess zinc and cadmium, we identified two positions in the genome where head-to-head oriented genes are both induced by metals. In both examples, a single predicted HZA enhancer is positioned between the two translational start sites. We hypothesized that a single enhancer can control both head-to-head genes, an arrangement that has not been extensively characterized. To test this hypothesis, we used CRISPR genome editing to precisely delete the HZAmT enhancer positioned between mtl-2 and T08G5.1; in this mutant, both head-to-head genes display severely reduced zinc-activated transcription, whereas zinc-activated transcription of more distant genes was not strongly affected. Deleting the HZAcF enhancer positioned between cdr-1 and F35E8.10 caused both head-to-head genes to display reduced cadmium-activated transcription, whereas cadmium-activated transcription of more distant genes was not strongly affected. These studies rigorously document that a single HZA enhancer can control two head-to-head genes, advancing our understanding of the diverse functions of enhancers. Article SummaryEnhancers are critical for gene expression, but a full understanding their functions has yet to be elucidated. We discovered two positions in the C. elegans genome where a pair of head-to-head oriented genes are both transcriptionally activated by excess zinc and/or cadmium; in both cases, one high-zinc activation (HZA) enhancer is positioned between the translation start sites. When these HZA enhancers were deleted by genome engineering, both head-to-head genes lost metal-activated transcription. These results demonstrate that a single HZA enhancer can control two head-to-head genes, advancing the understanding of enhancer function.