The presumptive somite boundary in the presomitic mesoderm (PSM) is defined by the anterior border of the expression domain of Tbx6 protein. During somite segmentation, the expression domain of Tbx6 is regressed by Ripply-meditated degradation of Tbx6 protein. Although the expression of zebrafish tbx6 remains restricted to the PSM, the transcriptional regulation of tbx6 remains poorly understood. Here, we show that the expression of zebrafish tbx6 is maintained by transcriptional autoregulation. We find that a proximallocated cis-regulatory module, TR1, which contains two putative Tbox sites, is required for somite segmentation in the intermediate body and for proper expression of segmentation genes. Embryos with deletion of TR1 exhibit significant reduction of tbx6 expression at the 12-somite stage, although its expression is initially observed. Additionally, Tbx6 is associated with TR1 and activates its own expression in the anterior PSM. Furthermore, the anterior expansion of tbx6 expression in ripply gene mutants is suppressed in a TR1dependent manner. The results suggest that the autoregulatory loop of zebrafish tbx6 facilitates immediate removal of Tbx6 protein through termination of its own transcription at the anterior PSM.

Somites sequentially form with a regular interval by the segmentation from the anterior region of the presomitic mesoderm (PSM). The expression of several genes involved in the somite segmentation is switched off at the transition from the anterior PSM to somites. Zebrafish Ripply1, which down-regulates a T-box transcription factor Tbx6, is required for the suppression of segmentation gene expression. However, the functional roles of the Ripply-mediated suppression of segmentation gene expression at the anterior PSM remain elusive. In this study, we generated ripply1 mutants and examined genetic interaction between ripply1/2 and tbx6. Zebrafish ripply1 embryos failed to form the somite boundaries as was observed in knockdown embryos. We found that somite segmentation defects in ripply1 mutants were suppressed by heterozygous mutation of tbx6 or partial translational inhibition of tbx6 by antisense morpholino. We further showed that somite boundaries that were recovered in tbx6; ripply1 embryos were dependent on the function of ripply2, indicating that relative gene dosage between ripply1/2 and tbx6 plays a critical role in the somite formation. Interestingly, the expression of segmentation genes such mesp as was still not fully suppressed at the anterior PSM of tbx6; ripply1 embryos although the somite formation and rostral-caudal polarity of somites were properly established. Furthermore, impaired myogenesis was observed in the segmented somites in tbx6; ripply1 embryos. These results revealed that partial suppression of the segmentation gene expression by Ripply is sufficient to establish the rostral-caudal polarity of somites, and that stronger suppression of the segmentation gene expression by Ripply is required for proper myogenesis in zebrafish embryos.

In vertebrates, the periodic formation of somites from the presomitic mesoderm (PSM) is driven by the molecular oscillator known as the segmentation clock. The hairy-related gene, hes6/her13.2, functions as a hub by dimerizing with other oscillators of the segmentation clock in zebrafish embryos. Although hes6 exhibits a posterior-anterior expression gradient in the posterior PSM with a peak at the tailbud, the detailed mechanisms underlying this unique expression pattern have not yet been clarified. By establishing several transgenic lines, we found that the transcriptional regulatory region downstream of hes6 in combination with the hes6 3'UTR recapitulates the endogenous gradient of hes6 mRNA expression. This downstream region, which we termed the PT enhancer, possessed several putative binding sites for the T-box and Ets transcription factors that were required for the regulatory activity. Indeed, the T-box transcription factor (Tbx16) and Ets transcription factor (Pea3) bound specifically to the putative binding sites and regulated the enhancer activity in zebrafish embryos. In addition, the 3'UTR of hes6 is required for recapitulation of the endogenous mRNA expression. Furthermore, the PT enhancer with the 3'UTR of hes6 responded to the inhibition of retinoic acid synthesis and fibroblast growth factor signaling in a manner similar to endogenous hes6. The results showed that transcriptional regulation by the T-box and Ets transcription factors, combined with the mRNA stability given by the 3'UTR, is responsible for the unique expression gradient of hes6 mRNA in the posterior PSM of zebrafish embryos.

In the zebrafish segmentation clock, hairy/enhancer of split‐related genes her1, her7, and hes6 encodes components of core oscillators. Since the expression of cyclic genes proceeds rapidly in the presomitic mesoderm (PSM), these hairy‐related mRNAs are subject to strict post‐transcriptional regulation. In this study, we demonstrate that inhibition of the CCR4‐NOT deadenylase complex lengthens poly(A) tails of hairy‐related mRNAs and increases the amount of these mRNAs, which is accompanied by defective somite segmentation. In transgenic embryos, we show that EGFP mRNAs with 3′UTRs of hairy‐related genes exhibit turnover similar to endogenous mRNAs. Our results suggest that turnover rates of her1, her7, and hes6 mRNAs are differently regulated by the CCR4‐NOT deadenylase complex possibly through their 3′UTRs in the zebrafish PSM.