Two-component signal-transduction systems in budding yeast MAP a different pathway?

Until recently, two-component signal-transduction pathways were thought to be exclusively found in bacteria. Some eukaryotic examples have now been characterized but, at least in the budding yeast Saccharomyces cerevisiae, it appears that this type of signal-transduction pathway is not utilized as extensively as in bacteria. Further, the few eukaryotic examples described suggest that two-component signal-transduction pathways might not be freestanding, as in prokaryotes, but might effect gene expression by regulating eukaryotic mitogen-activated protein (MAP) kinase pathways.

A yeast transcription factor bypassing the requirement for SBF and DSC1/MBF in budding yeast has homology to bacterial signal transduction proteins

The transcription factors SBF and DSC1/MBF bind SCB and MCB promoter elements, respectively, and are essential for the cell cycle progression of Saccharomyces cerevisiae through the control of G1 cyclin gene expression. We isolated a gene (BRY1; Bacterial Response regulator in Yeast) able to activate either MCB or SCB promoter elements on a reporter plasmid which, when overexpressed, can bypass the normally essential requirement for SBF and DSC1/MBF by the stimulation of CLN1 and CLN2 expression. In the case of CLN2 at least, this expression depends upon the MCB and SCB promoter elements. In wild-type yeast, the disruption of BRY1 has no apparent phenotype, but under conditions where the activities of SBF and DSC1/MBF are reduced, BRY1 becomes essential. Our data imply the existence of a third pathway affecting cyclin expression. BRY1 is the same gene as SKN7 which has significant sequence homology to the receiver domains found in response regulator proteins from the bacterial two-component signal transduction pathways. SKN7 is thought to affect cell wall structure, and when highly overexpressed we find that BRY1/SKN7 is lethal perhaps because of perturbations in cell wall biosynthesis. The lethality is partially rescued by genes from the protein kinase C pathway, but genetic data imply that BRY1/SKN7 and protein kinase C are not in the same pathway. Our results suggest that Bry1/Skn7 can influence the expression of MCB- and SCB-driven gene expression in budding yeast, perhaps including genes involved in cell wall metabolism, via a two-component signal transduction pathway which activates Bry1/Skn7 in response to an unidentified signal.

Sonic hedgehog is an endodermal signal inducing Bmp-4 and Hox genes during induction and regionalization of the chick hindgut

Reciprocal inductive signals between the endoderm and mesoderm are critical to vertebrate gut development. Sonic hedgehog encodes a secreted protein known to act as an inductive signal in several regions of the developing embryo. In this report, we provide evidence to support the role of Sonic hedgehog and its target genes Bmp-4 and the Abd-B-related Hox genes in the induction and patterning the chick hindgut. Sonic is expressed in the definitive endoderm at the earliest stage of chick gut formation. Immediately subjacent to Sonic expression in the caudal endoderm is undifferentiated mesoderm, later to become the visceral mesoderm of the hindgut. Genes expressed within this tissue include Bmp-4 (a TGF-beta relative implicated in proper growth of visceral mesoderm) and members of the Abd-B class of Hox genes (known regulators of pattern in many aspects of development). Using virally mediated misexpression, we show that Sonic hedgehog is sufficient to induce ectopic expression of Bmp-4 and specific Hoxd genes within the mesoderm. Sonic therefore appears to act as a signal in an epithelial-mesenchymal interaction in the earliest stages of chick hindgut formation. Gut pattern is evidenced later in gut morphogenesis with the presence of anatomic boundaries reflecting phenotypically and physiologically distinct regions. The expression pattern of the Abd-b-like Hox genes remains restricted in the hindgut and these Hox expression domains reflect gut morphologic boundaries. This finding strongly supports a role for these genes in determining the adult gut phenotype. Our results provide the basis for a model to describe molecular controls of early vertebrate hindgut development and patterning. Expression of homologous genes in Drosophila suggest that aspects of gut morphogenesis may be regulated by similar inductive networks in the two organisms.

Histone H4 and the maintenance of genome integrity

The normal progression of Saccharomyces cerevisiae through nuclear division requires the function of the amino-terminal domain of histone H4. Mutations that delete the domain, or alter 4 conserved lysine residues within the domain, cause a marked delay during the G2+M phases of the cell cycle. Site-directed mutagenesis of single and multiple lysine residues failed to map this phenotype to any particular site; the defect was only observed when all four lysines were mutated. Starting with a quadruple lysine-to-glutamine substitution allele, the insertion of a tripeptide containing a single extra lysine residue suppressed the G2+M cell cycle defect. Thus, the amino-terminal domain of histone H4 has novel genetic functions that depend on the presence of lysine per se, and not a specific primary peptide sequence. To determine the nature of this function, we examined H4 mutants that were also defective for G2/M checkpoint pathways. Disruption of the mitotic spindle checkpoint pathway had no effect on the phenotype of the histone amino-terminal domain mutant. However, disruption of RAD9, which is part of the pathway that monitors DNA integrity, caused precocious progression of the H4 mutant through nuclear division and increased cell death. These results indicate that the lysine-dependent function of histone H4 is required for the maintenance of genome integrity, and that DNA damage resulting from the loss of this function activates the RAD9-dependent G2/M checkpoint pathway.

Hox genes and the evolution of vertebrate axial morphology

A common form of evolutionary variation between vertebrate taxa is the different numbers of segments that contribute to various regions of the anterior-posterior axis; cervical vertebrae, thoracic vertebrae, etc. The term ‘transposition’ is used to describe this phenomenon. Genetic experiments with homeotic genes in mice have demonstrated that Hox genes are in part responsible for the specification of segmental identity along the anterior-posterior axis, and it has been proposed that an axial Hox code determines the morphology of individual vertebrae (Kessel, M. and Gruss, P. (1990) Science 249, 347–379). This paper presents a comparative study of the developmental patterns of homeobox gene expression and developmental morphology between animals that have homologous regulatory genes but different morphologies. The axial expression boundaries of 23 Hox genes were examined in the paraxial mesoderm of chick, and 16 in mouse embryos by in situ hybridization and immunolocalization techniques. Hox gene anterior expression boundaries were found to be transposed in concert with morphological boundaries. This data contributes a mechanistic level to the assumed homology of these regions in vertebrates. The recognition of mechanistic homology supports the historical homology of basic patterning mechanisms between all organisms that share these genes.

The activation of DNA replication in yeast

DNA replication in eukaryotic cells is initiated at sites in the DNA known as origins. Studies in yeast have identified a number of the genes and proteins that may be involved in this process. In this review, we concentrate largely on those genes on proteins that are required for initiation of DNA replication and for which there is some evidence for a role at origins.

Differential expression of myogenic regulatory genes and Msx-1 during dedifferentiation and redifferentiation of regenerating amphibian limbs

An amputated limb of an adult urodele amphibian is capable of undergoing regeneration. The new structures form from an undifferentiated mass of cells called the regenerative blastema. The cells of the blastema are believed to derive from differentiated tissues of the adult limb. However, the exact source of these cells and the process by which they undergo dedifferentiation are poorly understood. In order to elucidate the molecular and cellular basis for dedifferentiation we isolated a number of genes which are potential regulators of the process. These include Msx-1, which is believed to support the undifferentiated and proliferative state of cells in the embryonic limb bud; and two members of the myogenic regulatory gene family, MRF-4 and Myf-5, which are expressed in differentiated muscle and regulate muscle-specific gene activity. As anticipated, we find that Msx-1 is strongly up-regulated during the initiation of regeneration. It remains expressed throughout regeneration but is not found in the fully regenerated limb. The myogenic gene MRF-4 has the reverse expression pattern. It is expressed in adult limb muscle, is rapidly shut off in early regenerative blastemas, and is only reexpressed at the completion of regeneration. These kinetics are paralleled by those of a muscle-specific Myosin gene. In contrast Myf-5, a second member of the myogenic gene family, continues to be expressed throughout the regenerative process. Thus, MRF-4 and Myf-5 are likely to play distinct roles during regeneration. MRF-4 may directly regulate muscle phenotype and as such its repression may be a key event in dedifferentiation.(ABSTRACT TRUNCATED AT 250 WORDS)

Sonic hedgehog and Fgf-4 act through a signaling cascade and feedback loop to integrate growth and patterning of the developing limb bud

Proper limb growth and patterning requires signals from the zone of polarizing activity in the posterior mesoderm and from the overlying apical ectodermal ridge (AER). Sonic hedgehog and Fgf-4, respectively, have recently been identified as candidates for these signals. We have dissected the roles of these secreted proteins in early limb development by ectopically regulating their activities in a number of surgical contexts. Our results indicate that Sonic hedgehog initiates expression of secondary signaling molecules, including Bmp-2 in the mesoderm and Fgf-4 in the ectoderm. The mesoderm requires ectodermally derived competence factors, which include Fgf-4, to activate target gene expression in response to Sonic hedgehog. The expression of Sonic hedgehog and Fgf-4 is coordinately regulated by a positive feedback loop operating between the posterior mesoderm and the overlying AER. Taken together, these data provide a basis for understanding the integration of growth and patterning in the developing limb.

Comparison of native matrix metalloproteinases and their recombinant catalytic domains using a novel radiometric assay

A novel radiometric assay was developed for human fibroblast collagenase (matrix metalloproteinase-1, MMP-1), stromelysin (MMP-3), and their recombinant catalytic domains. Using this assay we were able to compare the native MMPs with the respective catalytic domains in terms of inhibitor affinities and peptide hydrolysis. The assay works on the same principle as an assay developed for carboxypeptidase (Rossier et al., Anal. Biochem. 1989, 178, 27-31) and is based on a synthetic peptide substrate, [1-benzoyl-14C]benzoyl-Pro-Leu-Ala-Leu-Trp- NH(CH2)4N(CH3)2(bnzPLALW-NX). The generation of product is measured by selective solvent extraction of radioactive product directly into scintillation cocktail; the entire assay, including the radioactivity measurement, is completed in a single 1-ml tube (96-well format) without removal or transfer of phases. Results of steady-state measurements demonstrated that peptide hydrolysis follows Michaelis-Menten kinetics with the fibroblast MMPs and their C-terminal deleted forms. The kinetic constants for hydrolysis of bnz-PLALW-NX, and for inhibition by actinonin, a natural peptide-hydroxamate, are essentially the same for the native and the C-terminally deleted MMPs.

The role of homeobox genes in limb development

Intensive study of the development of the vertebrate limb has led to a conceptual framework for understanding the specification of a limb primordium, the outgrowth of those cells and their organization and differentiation into a functional appendage. During the past few years, a number of homeobox-containing genes have been identified that are likely to play controlling roles in each of these events.

DNA synthesis control in yeast: An evolutionarily conserved mechanism for regulating DNA synthesis genes?

After yeast cells commit to the cell cycle in a process called START, genes required for DNA synthesis are expressed in late G1. Periodicity is mediated by a hexameric sequence, known as a MCB element, present in all DNA synthesis gene promoters. A complex that specifically binds MCBs has been identified. One polypeptide in the MCB complex is Swi6, a transcription factor that together with Swi4 also binds G1 cyclin promoters and participates in a positive feedback loop at START. The finding that Swi6 is directly involved in both START and DNA synthesis gene control suggest a model in which Swi6, activated through its participation in START, serves as the central transcription factor in coordinating late G1 gene expression. The mechanism may be conserved in all eukaryotic cells.

Targeted misexpression of Hox-4.6 in the avian limb bud causes apparent homeotic transformations

In the limb bud the 5' members of the Hox-4 gene cluster are expressed in a nested set of overlapping domains which are progressively restricted in the posterior and distal directions. These domains arise early in limb bud development and come to approximate the primordia of the major structural elements of the limb along the anterior/posterior axis (Fig. 1). This pattern, and the fact that surgical manipulations which lead to mirror image duplications along the anterior/posterior axis give rise to mirror image duplications of the domains of expression of these genes, have led to the proposal that these transcription factors specify positional identity along the anterior/posterior axis. Here we test this hypothesis directly using replication-competent retroviral vectors to expand the domain of expression of the Hox-4.6 gene anteriorly during limb development in vivo. We report that alteration of the domain of expression of the Hox-4.6 gene in the developing limb leads to reproducible pattern alterations consistent with a posterior homeotic transformation.

Functionally distinct isoforms of the CRE-BP DNA-binding protein mediate activity of a T-cell-specific enhancer

Expression of the CD3 delta gene of the T-cell receptor (TCR) complex is regulated by a T-cell-specific enhancer. A highly conserved 40-bp motif (element delta A) within the CD3 delta enhancer is responsible for mediating its activity and specificity. Element delta A exhibits sequence similarities to the cyclic AMP response element (CRE) but does not respond to changes in the level of cyclic AMP. Using the delta A element as a probe, we have isolated three cDNA clones encoding three distinct protein isoforms, products of differential splicing and alternate promoter usage of the CRE-BP gene. These isoforms share the DNA binding and dimerization domains at the C terminus of the protein but differ at their N termini. In transfection assays, their activities as transcription regulators differ: CRE-BP2 is a potent activator, CRE-BP3 is a weak activator, and CRE-BP1 is transcriptionally inert. Mutations in the basic region of the CRE-BP1 protein which abrogate its ability to bind DNA render this protein a dominant repressor of the delta A enhancer. Antibodies to the CRE-BP protein interact specifically with the ubiquitous and predominantly T-cell-restricted nuclear complexes that bind to the delta A element and suggest the presence of this protein in homo- and heterodimeric complexes. Since the delta A motif is also present in the enhancer and promoter of the TCR alpha and beta genes, the CRE-BP isoforms may mediate expression of other members of the CD3/TCR complex during T-cell development.

The highly conserved N-terminal domains of histones H3 and H4 are required for normal cell cycle progression

The N-terminal domains of the histones H3 and H4 are highly conserved throughout evolution. Mutant alleles deleted for these N-terminal domains were constructed in vitro and examined for function in vivo in Saccharomyces cerevisiae. Cells containing a single deletion allele of either histone H3 or histone H4 were viable. Deletion of the N-terminal domain of histone H4 caused cells to become sterile and temperature sensitive for growth. The normal cell cycle progression of these cells was also altered, as revealed by a major delay in progression through the G2 + M periods. Deletion of the N-terminal domain of histone H3 had only minor effects on mating and the temperature-sensitive growth of mutant cells. However, like the H4 mutant, the H3 mutants had a significant delay in completing the G2 + M periods of the division cycle. Double mutants containing N-terminal domain deletions of both histone H3 and histone H4 were inviable. The phenotypes of cells subject to this synthetic lethality suggest that the N-terminal domains are required for functions essential throughout the cell division cycle and provide genetic evidence that histones are randomly distributed during chromosome replication.

Genetic analysis of histone H4: essential role of lysines subject to reversible acetylation

The nucleosome is the fundamental unit of assembly of the chromosome and reversible modifications of the histones have been suggested to be important in many aspects of nucleosome function. The structure-function relations of the amino-terminal domain of yeast histone H4 were examined by the creation of directed point mutations. The four lysines subject to reversible acetylation were essential for histone function as the substitution of arginine or asparagine at these four positions was lethal. No single lysine residue was completely essential since arginine substitutions at each position were viable, although several of these mutants were slower in completing DNA replication. The simultaneous substitution of glutamine for the four lysine residues was viable but conferred several phenotypes including mating sterility, slow progression through the G2/M period of the division cycle, and temperature-sensitive growth, as well as a prolonged period of DNA replication. These results provide genetic proof for the roles of the H4 amino-terminal domain lysines in gene expression, replication, and nuclear division.

Adriamycin cardiomyopathy. Fatal outcome of general anaesthesia in a child with adriamycin cardiomyopathy

The death under general anaesthesia of a child with Adriamycin cardiomyopathy is reported. The acute, subacute and chronic cardiotoxic effects of Adriamycin are discussed and the risk factors for chronic Adriamycin cardiomyopathy presented, with particular reference to cumulative dosage of Adriamycin. The insidious onset of impairment of cardiac function is stressed and suggestions for anaesthetic management outlined.

Direct evidence for rifampicin-promoted readthrough of the partial terminator tL7 in the rpoBC operon of Escherichia coli

The RNA polymerase subunits beta and beta' of Escherichia coli, encoded by the genes rpoB and rpoC, are co-transcribed with four 50 S ribosomal protein genes, rplKAJL. After treatment with the antibiotic rifampicin a partial uncoupling of rpoBC from rplKAJL transcription occurs. We have been investigating the role played in uncoupling by tL7, an 80% efficient terminator of transcription present in the 319 bp intercistronic space between rplL and rpoB, using S1 nuclease mapping of transcripts produced in vivo in normal (rpoBC haploid) strains. Our results show directly that rifampicin stimulates readthrough of tL7 on the chromosome by approximately twofold, an effect sufficient to explain the observed increase in beta beta' protein synthesis. We also provide preliminary evidence for the map position of PL7, and show that both this and P beta, two very weak promoters which might in principle be activated by rifampicin, are not in fact stimulated by the drug.

Combined unilateral high frequency jet ventilation and contralateral intermittent positive pressure ventilation

The anaesthetic management of a patient who required right lower lobectomy for bronchial carcinoma associated with emphysema, pneumoconiosis and a previous thoracoplasty for pulmonary tuberculosis, is described. A technique of unilateral high frequency jet ventilation plus conventional intermittent positive pressure ventilation to the contralateral lung was used.