Biomedical subjects

Y B Shi

Publications and source records attributed to Y B Shi.

77 records · Page 5Linked to original sources

PubMed1987

The effects of covalent additions of a psoralen on transcription by E. coli RNA polymerase.

Synthetic DNA substrates containing a site-specifically engineered psoralen monoadduct or diadduct were used to characterize the response of the E. coli RNA polymerase elongation complex to these lesions. The psoralen derivative HMT (4'-hydroxymethyl-4,5', 8-trimethylpsoralen) was site specifically placed into two synthetic double-stranded DNA fragments each of which contained an E. coli RNA polymerase promoter at one end. The HMT molecule was attached to the middle of the DNA fragments as either a furan-side monoadduct or an interstrand diadduct. Transcription off the HMT crosslinked DNA templates showed that E. coli RNA polymerase terminated at the HMT diadduct site, i. e., one nucleotide before the modified thymidine residue on the template strand. The furan-side monoadduct when on the template strand also blocked transcription by the polymerase. However, no effect on transcription was observed when the monoadduct was located on the non-template strand.

Base Sequence↗

PubMed1987

Wavelength dependence for the photoreactions of DNA-psoralen monoadducts. 1. Photoreversal of monoadducts.

We have studied the wavelength dependence for the photoreversal of a monoadducted psoralen derivative, HMT [4'-(hydroxymethyl)-4,5',8-trimethylpsoralen], in a single-stranded deoxyoligonucleotide (5'-GAAGCTACGAGC-3'). The psoralen was covalently attached to the thymidine residue in the oligonucleotide as either a furan-side monoadduct, which is formed through the cycloaddition between the 4',5' double bond of the psoralen and the 5,6 double bond of the thymidine, or a pyrone-side monoadduct, which is formed through the cycloaddition between the 3,4 double bond of the psoralen and the 5,6 double bond of the thymidine. As a comparison, we have also investigated the wavelength-dependent photoreversal of the isolated thymidine-HMT monoadducts. All photoreversal action spectra correlate with the extinction spectra of the isolated monoadducts. In the case of the pyrone-side monoadduct, two absorption bands contribute to the photoreversal with a quantum yield of 2 X 10(-2) at wavelengths below 250 nm and 7 X 10(-3) at wavelengths from 287 to 314 nm. The incorporation of the monoadduct into the DNA oligomer had little effect upon the photoreversal rate. For the furan-side monoadduct at least three absorption bands contribute to the photoreversal. The quantum yield varied from 5 X 10(-2) at wavelengths below 250 nm to 7 X 10(-4) at wavelengths between 295 and 365 nm. In contrast to the case of the pyrone-side monoadduct, the incorporation of the furan-side monoadduct into the DNA oligomer reduced the photoreversal rate constant at wavelengths above 285 nm.

Cross-Linking Reagents↗

PubMed1987

Wavelength dependence for the photoreactions of DNA-psoralen monoadducts. 2. Photo-cross-linking of monoadducts.

The photoreactions of HMT [4'-(hydroxymethyl)-4,5',8-trimethylpsoralen] monoadducts in double-stranded DNA have been studied with complementary oligonucleotides. The HMT was first attached to the thymidine residue in the oligonucleotide 5'-GAAGCTACGAGC-3' as either a furan-side monoadduct or a pyrone-side monoadduct. The HMT-monoadducted oligonucleotide was then hybridized to the complementary oligonucleotide 5'-GCTCGTAGCTTC-3' and irradiated with monochromatic light. In the case of the pyrone-side monoadducted oligonucleotide, photoreversal was the predominant reaction, and very little cross-link was formed at all wavelengths. The course of the photoreaction of the double-stranded furan-side monoadducted oligonucleotide was dependent on the irradiation wavelength. At wavelengths below 313 nm, both photoreversal and photo-cross-linking occurred. At wavelengths above 313 nm, photoreversal of the monoadduct could not be detected, and photo-cross-linking occurred efficiently with a quantum yield of 2.4 X 10(-2).

Cross-Linking Reagents↗

PubMed1986

Wavelength dependence for the photoreversal of a psoralen-DNA cross-link.

We report an action spectrum for the photoreversal of a psoralen cross-link joining two self-complementary DNA oligonucleotides. The cross-link was formed between two thymines (T) on opposite strands of the DNA oligomers and 4'-(hydroxymethyl)-4,5',8-trimethylpsoralen (HMT). For comparison, we also present an action spectrum for the photoreversal of the isolated diadduct T-HMT-T. The wavelength dependence for the diadduct photoreversal parallels its absorption spectrum. Both the diadduct and the cross-linked DNA can be photoreversed by exposure to light with wavelengths between 240 and 313 nm. We did not observe photoreversal at 334 nm or above. At least two distinct absorption bands appear to contribute to photoreversal. We measured a quantum yield of 0.16 for photoreversal of the isolated diadduct at wavelengths between 240 and 266 nm. For wavelengths above 280 nm, the quantum yield is 0.30. We also observed a preferential photoreversal at the furan end of the psoralen in the T-HMT-T diadduct. In contrast, the cross-linked DNA oligonucleotides preferentially photoreversed at the pyrone end of the psoralen adduct. The rate constant for photoreversal of the cross-linked DNA is larger than that for the isolated diadduct at wavelengths below 300 nm.

Base Sequence↗

PubMedDate not supplied

Thyroid hormone regulation of Xenopus laevis metamorphosis: functions of thyroid hormone receptors and roles of extracellular matrix remodeling.

The regulatory effects of the thyroid hormone on amphibian metamorphosis is mediated by thyroid hormone receptors. Using Xenopus laevis as a model system, we and others have shown that the mRNA levels of thyroid hormone receptors and 9-cis retinoic acid receptors, which form the functional heterodimers with thyroid hormone receptors, are regulated temporally in a tissue-dependent manner so that high levels of their mRNAs are present in an organ when metamorphosis is occurring. By overexpressing thyroid hormone receptors, 9-cis retinoic acid receptors, or both into developing Xenopus embryos, we have shown that both thyroid hormone receptors and 9-cis retinoic acid receptors are required for mediating the effects of thyroid hormone on embryo development and precocious but specific regulation of the genes, which are normally regulated by thyroid hormone during metamorphosis. Analyses of the developmental expression of one class of thyroid hormone response genes, which encode extracellular matrix-degrading metalloproteinases, suggest that extra cellular remodeling plays an important role during tissue remodeling, including cell death (apoptosis) and cell proliferation and differentiation. This effect of extracellular matrix on cell behavior has been supported directly by in vitro primary cell culture experiments, in which intestinal epithelial cells undergo thyroid hormone-induced apoptosis, just like that during natural metamorphosis.

Animals↗