Transcriptional cascades that specify cell fate have been well described in invertebrates. In mammalian development, however, gene hierarchies involved in determination of cell lineage are not understood. With the recent cloning of the MyoD family of myogenic regulatory factors, a model system has become available with which to study the dynamics of muscle determination in mammalian development. Myogenin, along with other members of the MyoD gene family, possesses the apparent ability to redirect nonmuscle cells into the myogenic lineage. This ability appears to be due to the direct activation of an array of subordinate or downstream genes which are responsible for formation and function of the muscle contractile apparatus. Myogenin-directed transcription has been shown to occur through interaction with a DNA consensus sequence known as an E box (CANNTG) present in the control regions of numerous downstream genes. In addition to activating the transcription of subordinate genes, members of the MyoD family positively regulate their own expression and cross-activate one another's expression. These autoregulatory interactions have been suggested as a mechanism for induction and maintenance of the myogenic phenotype, but the molecular details of the autoregulatory circuits are undefined. Here we show that the myogenin promoter contains a binding site for the myocytespecific enhancer-binding factor, MEF-2, which can function as an intermediary of myogenin autoactivation. Since MEF-2 can be induced by myogenin, these results suggest that myogenin and MEF-2 participate in a transcriptional cascade in which MEF-2, once induced by myogenin, acts to amplify and maintain the myogenic phenotype by acting as a positive regulator of myogenin expression.The formation of skeletal muscle during vertebrate development involves the induction of mesoderm from primary ectoderm and the subsequent generation of proliferating myoblasts that ultimately terminally differentiate in response to environmental cues. The recent discovery of a family of related muscle-specific factors that can convert fibroblasts to myoblasts has contributed to rapid progress toward understanding the molecular events that underlie the establishment of the skeletal muscle phenotype (for reviews, see references 55 and 69). Members of this muscle regulatory gene family include MyoD (21), myogenin (25, 77), myf5 (9), and MRF4/herculin/myf6 (8,48,61), each of which can activate myogenesis when introduced into a wide range of nonmuscle cell types.

Myogenin is a muscle-specific nuclear factor that acts as a genetic switch to activate myogenesis. Myogenin, MyoD, and a growing number of proteins implicated in transcriptional control share sequence homology within a basic region and an adjacent helix-oop-helix motif. Here we identify by site-directed mutagenesis a 12-amino acid subdomain of the myogenin basic region essential for binding of DNA and activation of myogenesis. The basic region of the widely expressed helix-oop-helix protein E12 is conserved at 8 of these 12 residues and can mediate DNA binding when placed in myogenin, but it cannot activate myogenesis. Replacement of each of the four nonconserved residues of the myogenin basic region with the corresponding residues of E12 reveals two adjacent amino acids (AlassThr87) that can impart muscle specificity to the basic region. These residues are specific to, and conserved in, the basic regions of all known myogenic helix-oop-helix proteins from Drosophil to man, suggesting that they constitute part of an ancient protein motif required for activation of the myogenic program.

Details of the total synthesis of rapamycin (1) are reported. The synthesis required the preparation of intermediates 4-9 in nonracemic form; key coupling reactions included a chromium-mediated addition of vinyl iodide 8 to aldehyde 7 and an Evans aldol reaction to couple fragments 62 and 9. Intermediates 4 and 6 were joined through an amide bond formation to afford advanced intermediate 71. Swern oxidation of the diol in 71 was

Myogenin is a skeletal muscle-specific transcription factor that can activate myogenesis when introduced into a variety of nonmuscle cell types. Activation of the myogenic program by myogenin is dependent on its binding to a DNA sequence known as an E box, which is associated with numerous muscle-specific genes. Myogenin shares homology with MyoD and other myogenic regulatory factors within a basic region and a helix-loop-helix (HLH) motif that mediate DNA binding and dimerization, respectively. Here we show that the basic region-HLH motif of myogenin alone lacks transcriptional activity and is dependent on domains in the amino and carboxyl termini to activate transcription. Analysis of these N-and C-terminal domains through creation of chimeras with the DNA-binding domain of the Saccharomyces cerevisiae transcription factor GAL4 revealed that they act as strong transcriptional activators. These transcription activation domains are dependent for activity on a specific amino acid sequence within the basic region, referred to as the myogenic recognition motif (MRM), when an E box is the target for DNA binding. However, the activation domains function independent of the MRM when DNA binding is mediated through a heterologous DNA-binding domain. The activation domain of the acidic coactivator VP16 can substitute for the myogenin activation domains and restore strong myogenic activity to the basic region-HLH motif. Within a myogenin-VP16 chimera, however, the VP16 activation domain also relies on the MRM for activation of the myogenic program. These findings reveal that DNA binding and transcriptional activation are separable functions, encoded by different domains of myogenin, but that the activity of the transcriptional activation domains is influenced by the DNA-binding domain. Activation of muscle-specific transcription requires collaboration between the DNA-binding and activation domains of myogenin and is dependent on events in addition to DNA binding.Differentiation of skeletal myoblasts to terminally differentiated myotubes is associated with the transcriptional activation of a set of genetically unlinked muscle-specific genes that encode proteins required for the specialized functions of the myofiber. Myogenin is a nuclear phosphoprotein that belongs to a family of skeletal musclespecific regulatory factors that can activate the muscle differentiation program when expressed artificially in a variety of nonmuscle cell types (15,50 Activation of muscle-specific transcription by members of the MyoD family is dependent on their binding to a conserved DNA sequence motif known as an E box (CANNTG), which is located in the regulatory regions of many muscle-specific genes, including MCK (6,20,22,41), MLC1/3 (49), acetylcholine receptor a-subunit (34), cardiac a-actin (36), and troponin I (27). Myogenic HLH proteins bind weakly to the E-box consensus sequence as homooli-* Corresponding author. gomers in vitro, but their affinity for this site increases dramatically upon heterooligomerization with the ubiquitous HLH p...

Details of the total synthesis of rapamycin (1) are reported. The synthesis required the preparation of intermediates 4-9 in nonracemic form; key coupling reactions included a chromium-mediated addition of vinyl iodide 8 to aldehyde 7 and an Evans aldol reaction to couple fragments 62 and 9. Intermediates 4 and 6 were joined through an amide bond formation to afford advanced intermediate 71. Swern oxidation of the diol in 71 was (USA) 0 V C H Verlugsgesellsrhufr mbH. 049451 Weinhrim. 1995 OS?OO833/95~0SO5-03~8 $ SO.W+ .25/0 Chem. Eur. 1 1995. I, No. S 318-333macrolactamization or macrolactonization strategies as demonstrated in the synthesis of the related natural product FK506 by the MerckI"] and the Harvard[''] groups. However, careful inspection of the structure of rapamycin suggested to us the possibility of constructing the macrocyclic ring by inserting a C 19-C20 ethene unit between two terminal vinyl iodides to form simultaneously the triene ahd the 31 -membered macrocyclic systems (Scheme 1). It was also anticipated that this "stitching cyclization" could be achieved after removal of all protecting groups and adjustment of oxidation states; this last step would then deliver rapamycin (1) directly.Disconnection of the triene system in 1 (Scheme 1, Stille palladium-catalyzed coupling[' 91) suggests bis(viny1 iodide) 2 and distannylethene 3 I 2 O * 211 (C 19-C20 fragment) as potential precursors. Further disconnection of the indicated amide and ester bonds in 2 and opening of the lactol ring reveals, upon appropriate functional group adjustments, compounds 4-6 as advanced key intermediates. The most complex of the latter three fragments, compound 6 was then dissected (Evans aldol reaction for the C 34-C 35 bond and a chromium -nickel coupling[221 for the C 28 -C 29 bond) to afford, after functional group manipulations, compounds 7-9 as potential building blocks. Thus a strategy was devised entailing construction and coupling of intermediates 7-9 and final elaboration to rapamycin.Total Synthesis of Rapamycin: The first task in the projected total synthesis of rapamycin was the construction of the key building blocks 4, 7, 8, and 9. These compounds were synthesized from readily available starting materials and by means of highly stereocontrolled sequences. istry. Nicolaou's research interests focus on chemical synthesis, molecular design and molecular recognition, and the biological acrions ofmolecules. He is the author or co-author of over 330 publications, 45 patents, and two books. Nicolaou's current research includes chemical synthesis. molecular design, and biological evaluation of compounds,from the areas of taxoids, enediynes. carbohydrates, D N A interacting molecules, DNA replication, zaragozic acids, and brevetoxins. Chem. Eiir. J. 1995. I. No. 5 j) 0,. MeOH/CH,CI, (1:l). -78°C. then Me,S (5.0equiv), -78-25"C. 16h, 88%; k) 57 (1.5 equiv), LiCl (2.0 equiv). N.N-diisopropylethylamine (2.0 equiv). then 56 (1.0 equiv). 25 "C. 6 h, 96%; I) Rh(PPh,),CI (0.05 equiv). Et,SiH. 50°C. 2 h, then aq. H F in CH,C...

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Myogenin is a muscle-specific transcription factor that can activate myogenesis; it belongs to a family of transcription factors that share homology within a basic region and an adjacent helix-loop-helix (HLH)

Arsenic in groundwater has been found above the WHO maximum permissible limit of 0.05 mg 1-1 in six districts of West Bengal, India covering an area of 34000 km2 with a population of 30 million. At present, 37 administrative blocks by the side of the river Ganga and adjoining areas are affected, as shown in Fig. 1. More than 800000 people of 312 villages/wards are drinking arsenic contaminated water and amongst them at least 175 000 people are showing arsenical skin lesions which are late manifestations of arsenic toxicity.