RY 1 XAIR￥1 Hλ PP D30Q9A二二二P c=HATR了1 PG8-------------- 基A￥12吉5 TR工BHA工RY1 释 I RY 1 Se-e-- HAI RY I m52:!你 E VIORVPMEOO PL S LVIKK SE c·HAIR1 H粪IRY1 恚灘 Figure 1. Sequence Analysis of c-hairy7 and the insect hairy protein 二 highest homology with the Xenopus x-hairy he zebrafish Hero, and the mammalian He eads) and the orange do 1996)(between white arrowheads) are we as the tetrapeptide WRPW at the carboxyl terminus, essential to recruit the corepressor Groucho and exert its negative effect on the transcriptional apparatus(Paroush et al., 1994 that blocking protein synthesis in embryo explants leads screen a random-primed CDNA library prepared from to an arrest of somitogenesis but that the oscillations chick embryonic mRNA, and several positive clones of c-hairy1 expression persist. This provides evidence were isolated. Sequence analysis of a fraction of these against the cyclic c-hairy 1 expression being under nega CDNAs revealed that they arise from a new gene, named tive autoregulatory control. Together, these results dem C-hairy. Comparison with other vertebrate Hairy-like onstrate that cells of the PSM undergo a defined and genes reveals that c-hairy] is most similar to the Xeno- constant number of c-hairy1 expression cycles between pus laevis hairy, the mammalian HES, and the zebrafish emergence from the primitive streak and incorporation Herb genes( Figure 1) into a somite. The rhythmic oscillations of the c-hairyl The putative c-hairy1 protein is 291 amino acids long ssenger RNA in prospective somitic cells provide the including a bHLH domain and the tetrapeptide WRPw first molecular evidence in favor of a developmental at the carboxyl terminus, which are characteristic fea clock involved in vertebrate segmentation ures of the hairy-related class of bHLH transcription factors in flies and vertebrates(Figures 1 and 2). Analysis Results of the c-hairy1 sequence suggests that it belongs to a ubgroup of the WRPw-containing bHLH proteins. Identification of an Avian hairy Homolog(c-hairyn) which includes mammalian HESI and HES2, Xenopus Expressed in the Paraxial Mesoderm X-hairy1, zebrafish Her6, and the fly and tribolium ha To identify chick homologs of the fly pair-rule gene he (Figure 1). The Enhancer-of-split and the zebrafish Her1 we used a PCR-based approach with degenerate oligo- genes are only distantly related to these hairy-like g nucleotides that correspond to sequences conserved( Figure 2). In Drosophila, these proteins act as transcrip- between the two hairy-like genes in Drosophila (hairy tional repressors in a variety of developmental contexts and deadpan). An initial PCR fragment was used to(Ohsako et al. 1994; Paroush et al. 1994: Van Doren etCell 640 Figure 1. Sequence Analysis of c-hairy1 Comparison of the c-hairy1 protein sequence with that of other vertebrate homologs belonging to the Hairy/Enhancer-of-split (HES) family and the insect hairy proteins using the ClustalX programme (Higgins et al., 1996). c-hairy1 shows highest homology with the Xenopus x-hairy1, the zebrafish Her6, and the mammalian HES genes. The bHLH domain (between black arrowheads) and the orange domain (Fisher et al., 1996) (between white arrowheads) are well conserved between all the HES proteins, as well as the tetrapeptide WRPW at the carboxyl terminus, essential to recruit the corepressor Groucho and exert its negative effect on the transcriptional apparatus (Paroush et al., 1994; Fisher et al., 1996). that blocking protein synthesis in embryo explants leads screen a random-primed cDNA library prepared from to an arrest of somitogenesis but that the oscillations chick embryonic mRNA, and several positive clones of c-hairy1 expression persist. This provides evidence were isolated. Sequence analysis of a fraction of these against the cyclic c-hairy1 expression being under nega- cDNAs revealed that they arise from a new gene, named tive autoregulatory control. Together, these results dem- c-hairy1. Comparison with other vertebrate Hairy-like onstrate that cells of the PSM undergo a defined and genes reveals that c-hairy1 is most similar to the Xeno￾constant number of c-hairy1 expression cycles between pus laevis hairy1, the mammalian HES, and the zebrafish emergence from the primitive streak and incorporation Her6 genes (Figure 1). into a somite. The rhythmic oscillations of the c-hairy1 The putative c-hairy1 protein is 291 amino acids long, messenger RNA in prospective somitic cells provide the including a bHLH domain and the tetrapeptide WRPW first molecular evidence in favor of a developmental at the carboxyl terminus, which are characteristic fea￾clock involved in vertebrate segmentation. tures of the hairy-related class of bHLH transcription factors in flies and vertebrates (Figures1 and 2). Analysis Results of the c-hairy1 sequence suggests that it belongs to a subgroup of the WRPW-containing bHLH proteins, Identification of an Avian hairy Homolog (c-hairy1) which includes mammalian HES1 and HES2, Xenopus Expressed in the Paraxial Mesoderm X-hairy1, zebrafish Her6, and the fly and tribolium hairy To identify chick homologs of the fly pair-rule gene hairy, (Figure 1). The Enhancer-of-split and the zebrafish Her1 we used a PCR-based approach with degenerate oligo- genes are only distantly related to these hairy-like genes nucleotides that correspond to sequences conserved (Figure 2). In Drosophila, these proteins act as transcrip￾between the two hairy-like genes in Drosophila (hairy tional repressors in a variety of developmental contexts and deadpan). An initial PCR fragment was used to (Ohsako et al., 1994; Paroush et al., 1994; Van Doren et