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T Mohun

Publications and source records attributed to T Mohun.

24 records · Page 2Linked to original sources
PubMed1987

Xenopus cytoskeletal actin and human c-fos gene promoters share a conserved protein-binding site.

Xenopus laevis cytoskeletal actin gene promoters contain a 20-bp sequence homologous to the serum response element (SRE) required for transient human c-fos gene transcription in response to serum factors. Both sequences bind the same factor in HeLa cell extracts, as shown by binding competition, DNase I and dimethylsulphate (DMS) protection and DMS interference assays. A similar protein is present in Xenopus laevis oocytes. Sequences containing the SRE homology are essential for constitutive activity of the actin promoter in both Xenopus and mouse cells, and a synthetic SRE functions as a promoter element in these cells. In mouse cells, transcription of both transfected Xenopus actin and actin/c-fos fusion genes is activated following serum stimulation. These data suggest that the SRE and its cognate protein form part of a regulatory pathway that has been highly conserved during evolution.

Actins↗
PubMed1984

Evolution of the human sarcomeric-actin genes: evidence for units of selection within the 3' untranslated regions of the mRNAs.

The complete 3' untranslated region (3'UTR) sequence of the human skeletal-actin gene has been compared with the corresponding regions of the rat and chicken skeletal-actin genes. This comparison reveals that the skeletal-actin 3'UTR is composed of conserved and nonconserved segments. By using genomic Southern transfer blots and thermal stability (Tm) measurements, we found that the cardiac-actin gene 3'UTR also consists of conserved and nonconserved segments. Comparison of human and Xenopus laevis cardiac-actin mRNA sequences confirms the presence of a region of high similarity in the 3'UTR. We conclude that subsegments of the 3'UTRs of both skeletal- and cardiac-actin genes of birds and mammals are under considerable selective pressure. This suggests that these conserved sequences may have functional roles in actin-gene expression or regulation, and that these roles might be different for each actin isoform.

Actins↗
PubMed1983

Distinct organizations and patterns of expression of early and late histone gene sets in the sea urchin.

The set of histone genes that are active late in embryogenesis of the sea urchin (Strongylocentrotus purpuratus) are present in 5-12 copies per genome and, unlike the clustered, tandemly arrayed early histone genes, are dispersed and irregularly arranged. Late H2B gene expression is activated by events accompanying fertilization and its mRNAs a, first detectable by as early as 6 h of development (16 cells) and increase only slightly in amount during the period of rapid cleavage between 6 and 14 h. However, during the short interval between 14 and 16 h, while the amount of early histone mRNA is declining, there is a greater than 15-fold burst in the rate of late H2B mRNA accumulation.

Animals↗
PubMed1981

The influence of topology and glycosylation on the fate of heterologous secretory proteins made in Xenopus oocytes.

Secretory proteins made in Xenopus laevis oocytes under the direction of heterologous messenger RNA are modified, topologically segregated and exported. Thus the oocyte may serve as a useful surrogate secretory system and we have studied some of the factors governing access to the export pathway. Unglycosylated chicken ovalbumin, synthesized and trapped in the cytosol, is not secreted but glycosylated ovalbumin, found sequestered within vesicles, is exported from oocytes. However, ovalbumin, which is transferred across the endoplasmic reticulum in the presence of tunicamycin and which is indistinguishable by immunoprecipitation, by two-dimensional gel electrophoresis and by concanavalin-A--Sepharose binding from the cytosolic form, is still secreted. Guinea-pig milk proteins and human interferon are also exported from tunicamycin-treated frog cells. These observations demonstrate that access to the endoplasmic reticulum but not glycosylation is a mandatory intermediate step in secretion, and emphasize the advantages of the oocyte as a surrogate system for the study of the later events in the gene expression pathway.

Animals↗
PubMed1980

The Xenopus oocyte as a surrogate secretory system. The specificity of protein export.

Combining messenger RNA from one kind of secretory cell with the cytoplasm of another such cell can reveal the nature and specificity of protein export mechanisms. We show that messenger RNAs from secretory cells of chickens, rats, mice, frogs, guinea-pigs, locusts and barley plants, when injected into Xenopus oocytes, direct the synthesis and export of proteins. Chicken ovalbumin, Xenopus albumin, mouse thyroid-stimulating hormone, locust vitellin and guinea-pig milk proteins were identified using specific antibodies, whilst chicken lysozyme and ovomucoid, rat albumin, Xenopus vitellogenin and rat seminal vesicle basic proteins were identified provisionally from their molecular weights. Certain endogenous proteins are sequestered and secreted although most oocyte proteins are not exported. Similarly the major polyoma viral protein and the simian virus 40 and polyoma tumour antigens are retained within the oocyte. Radioactive proteins exported by oocytes programmed with chicken oviduct or Xenopus liver RNA are not re-exported in detectable amounts when injected into fresh oocytes, nor is there secretion of chicken oviduct or guinea-pig mammary gland primary translation products prepared using wheat germ extracts. Thus the export of secretory proteins from oocytes cannot be explained by leakage and may require a cotranslational event. The secretory system of the oocyte is neither cell-type nor species-specific yet is highly selective. We suggest that the oocyte can be used as a general surrogate system for the study of gene expression, from transcription through translation to the final subcellular or extracellular destination of the processed protein.

Animals↗