A novel transcription factor Rwdd1 and its SUMOylation inhibit the expression of sqr, a key gene of mitochondrial sulfide metabolism in Urechis unicinctus.

Sulfide-quinone oxidoreductase (SQR) is a key enzyme of sulfide metabolism in metazoans, and responsible for oxidizing sulfide into thiosulfate and transmitting the generated electrons to the ubiquinone. It has been revealed that the sqr mRNA level increases significantly in echiuran worm Urechis unicinctus exposed to sulfide, and HSF1, NF1 and Sp1 have been verified to participate in its transcriptional regulation. In this study, we obtained 23 potential transcription factors interacting possibly with the proximal region (-391 to +50) of sqr promoter, and focused on the RWD domain-containing 1 (Rwdd1), a protein with the maximum number of clones in yeast one-hybrid (Y1H) screening, to investigate its transcriptional regulation to U. unincitus sqr. The ChIP and EMSA assays identified that the Rwdd1 can bind directly to the promoter (+18/+36) of U. unicinctus sqr. The point mutation and transient transfection experiments discovered that TACG was the key sequence of the DNA element bound by the Rwdd1. Furthermore, the U. unicinctus Rwdd1 (UuRwdd1) was identified to be a transcription repressor inhibiting the sqr promoter activity, and the SUMOylation of UuRwdd1 at the lysine of 90th enhanced its inhibitory effect on sqr transcription further. Western blotting found Rwdd1 responded to sulfide in hindguts from U. unincitus, and the protein content showed a remarkable drop in hindgut nuclei in the early sulfide exposure, and then increased significantly both in the total protein and the nuclear protein extract. We suggested that the Rwdd1 is a novel transcription factor, and these data improve our understanding of the sqr transcriptional regulation and the mitochondrial sulfide metabolism.