The octamer motif is a common cis-acting regulatory element that functions in the transcriptional control regions of diverse genes and in viral origins of replication. The , were present at low levels in oocytes and early embryos and were dramatically upregulated during early gastrulation. In contrast to the Oct-60 mRNA, translation of Oct-25 mRNA appeared to be developmentally regulated, since the corresponding protein was detected in embryos during gastrulation but not in oocytes or rapidly cleaving embryos. Transcripts from the third POU protein gene, Oct-91, were induced after the midblastula transition and reached their highest levels of accumulation during late gastrulation. The expression of all three genes decreased during late gastrulation and early neurulation. By analogy with other members of the POU-domain gene family, the products of these genes may play critical roles in the determination of cell fate and the regulation of cell proliferation.

Members of the MyoD family of helix-loop-helix proteins control expression of the muscle phenotype by regulating the activity of subordinate genes. To investigate processes that control the expression of myogenic factors and regulate the establishment and maintenance of the skeletal muscle phenotype, we have analyzed sequences necessaxy for transcription of the maternally expressedXenopus MyoD (XMyoD) gene. A 3.5-kb DNA fragment containing theXMyoDa promoter was expressed in a somite-specific manner in injected frog embryos.The XMyoDa promoter was active in oocytes and cultured muscle cells but not in fibroblasts or nonmuscle cell lines. A 58-bp fragment containing the transcription initiation site, a GC-rich region, and overlapping binding sites for .the general transcription factor TFIID and the muscle-specific factor MEF2 was sufficient for muscle-specific transcription. Transcription of the minimalXMyoDa promoter in nonmuscle cells was activated by expression ofXenopus MEF2 (XMEF2) and required binding of both MEF2 and TFIID to the TATA motif. These results demonstrate that theXMyoDa TATA motif is a target for a cell-type-specific regulatory factor and suggests that MEF2 stabilizes and amplifies XMWyoDa transcription in mesodermal cells committed to the muscle phenotype.MyoD and other myogenic factors of the basic helix-loophelix (bHLH) protein family are thought to control the differentiation of skeletal muscle cells in culture (reviewed in references 32 and 50). Forced expression of myogenic factors in most cells positively regulates the expression of a set of downstream muscle-specific genes and activates the myogenic pathway. The MyoD family of myogenic regulatory factors appears to reside in a regulatory gene hierarchy and controls the muscle phenotype by binding to a subset of sequences containing the CANNTG (or E-box) consensus motif present in transcriptional regulatory regions of certain muscle-specific genes.Differentiated skeletal muscle cells also express members of the recently described MEF2 gene family that encode proteins containing a conserved DNA-binding domain, termed the MADS box, and a MEF2-specific domain of unknown function (33, 57). Upon forming homo-and heterodimeric complexes among themselves, proteins encoded by the MEF2 family interact with a subset of A+T-rich sequences present in muscle-specific enhancer and promoter elements (18,33). Expression of the MEF2 gene family, unlike that of the myogenic HLH gene family, is not restricted to skeletal muscle but occurs in cardiac and smooth muscle as well as in the brain (3, 57).Although much progress has been made regarding an understanding of the function of myogenic regulatory factors, relatively little is known about the mechanisms that control their expression. In view of the highly restricted expression of these genes in adult tissues and their profound effects on cellular phenotypes when expressed in many nonmuscle cells, it is expected that their expression is

No abstract

The octamer motif is a common cis-acting regulatory element that functions in the transcriptional control regions of diverse genes and in viral origins of replication. The ability of a consensus octamer motif to stimulate transcription of a histone H2B promoter in frog oocytes suggests that oocytes contain a transcriptionally active octamer-binding protein(s). We show here that frog oocytes and developing embryos contain multiple octamer-binding proteins that are expressed in a sequential manner during early development. Sequences encoding three novel octamer binding-proteins were isolated from Xenopus cDNA libraries by virtue of their homology with the DNA binding (POU) domain of Oct-1. The predicted POU domains of these proteins were most highly related to mammalian Oct-3 (also termed Oct-4), a germ line-specific gene required for mouse early development. Transcripts from these amphibian POU-domain genes were most abundant during early embryogenesis and absent from most adult somatic tissues. One of the genes, termed Oct-60, was primarily expressed as a maternal transcript localized in the animal hemisphere in mature oocytes. The protein encoded by this gene was present in oocytes and early embryos until the gastrula stage of development. Transcripts from a second POU-domain gene, Oct-25, were present at low levels in oocytes and early embryos and were dramatically upregulated during early gastrulation. In contrast to the Oct-60 mRNA, translation of Oct-25 mRNA appeared to be developmentally regulated, since the corresponding protein was detected in embryos during gastrulation but not in oocytes or rapidly cleaving embryos. Transcripts from the third POU protein gene, Oct-91, were induced after the midblastula transition and reached their highest levels of accumulation during late gastrulation. The expression of all three genes decreased during late gastrulation and early neurulation. By analogy with other members of the POU-domain gene family, the products of these genes may play critical roles in the determination of cell fate and the regulation of cell proliferation.

Members of the MyoD family of helix-loop-helix proteins control expression of the muscle phenotype by regulating the activity of subordinate genes. To investigate processes that control the expression of myogenic factors and regulate the establishment and maintenance of the skeletal muscle phenotype, we have analyzed sequences necessary for transcription of the maternally expressed Xenopus MyoD (XMyoD) gene. A 3.5-kb DNA fragment containing the XMyoDa promoter was expressed in a somite-specific manner in injected frog embryos. The XMyoDa promoter was active in oocytes and cultured muscle cells but not in fibroblasts or nonmuscle cell lines. A 58-bp fragment containing the transcription initiation site, a GC-rich region, and overlapping binding sites for the general transcription factor TFIID and the muscle-specific factor MEF2 was sufficient for muscle-specific transcription. Transcription of the minimal XMyoDa promoter in nonmuscle cells was activated by expression of Xenopus MEF2 (XMEF2) and required binding of both MEF2 and TFIID to the TATA motif. These results demonstrate that the XMyoDa TATA motif is a target for a cell-type-specific regulatory factor and suggests that MEF2 stabilizes and amplifies XMyoDa transcription in mesodermal cells committed to the muscle phenotype.