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M Han

Publications and source records attributed to M Han.

192 records · Page 11Linked to original sources
PubMed1988

Effects of histone H4 depletion on the cell cycle and transcription of Saccharomyces cerevisiae.

We have constructed a yeast strain (UKY403) in which the sole histone H4 gene is under control of the GAL1 promoter. This allows the activation of H4 mRNA synthesis on galactose and its repression on glucose. UKY403 cells, pre-synchronized in G1 with alpha-mating factor, have been used to show that glucose treatment results in the loss of approximately half the chromosomal nucleosomes. This depletion is only partially reversible when the H4 gene is reactivated on galactose. It was found that the resultant lethality manifests itself first in S phase, the period of nucleosome assembly, but leads to highly synchronous arrest in G2 and a virtually complete block in chromosomal segregation. Histone H4-depleted chromatin was analyzed for its efficiency as a template for all three RNA polymerases. Using pulse-labeling, we find no evidence for altered transcription by RNA polymerase I (25S, 18S and 5.8S rRNAs) or RNA polymerase III (5S rRNA, tRNAs). Northern blot analysis was used to measure levels of RNA polymerase II transcripts. There was little effect on the activation or repression of the CUP1 chelatin gene. While there may be some decrease in the level of certain mRNAs (e.g. HIS4, ARG4) other message levels (HIS3, TRP1) show little change upon glucose repression. Therefore, nucleosome loss certainly does not have a general effect on transcription.

Cell Cycle↗
PubMed1988

Depletion of histone H4 and nucleosomes activates the PHO5 gene in Saccharomyces cerevisiae.

We have previously constructed a yeast strain (UKY403) whose sole histone H4 gene is under control of the GAL1 promoter. This yeast arrests in G2 upon glucose treatment as a result of histone H4 depletion. The yeast PHO5 gene contains phase nucleosomes covering promoter (UAS) sequences in the PHO5 repressed state and it has been suggested that nucleosomes prevent the binding of positively acting factors to these UAS sequences. Using UKY403 we examined the length of polynucleosomes and nucleosome phasing in the PHO5 upstream region by the use of micrococcal nuclease and indirect end-labeling. It was found that glucose arrest led to a severe disruption in PHO5 chromatin structure and that most nucleosomes had their position altered or were lost from the PHO5 promoter region. Cell undergoing nucleosome depletion synthesized large quantities of accurate PHO5 transcripts even under repressive, high inorganic phosphate conditions. Histone H4 depletion did not appear to affect the repression or activation of another inducible yeast gene, CUP1. Arrest with landmarks in early G1 (in the cell division cycle mutant cdc28) or in various stages of G2 (in cdc15, cdc17 and cdc20) does not activate PHO5; nor does arrest due to chromosome topology changes (in top2 or the top1top2 topoisomerase mutants). cdc14, which has its arrest landmark at a similar point in the cell cycle as cdc15, does derepress PHO5. However, since it also leads to derepression of CUP1 it is probably functioning through an independent mechanism. Therefore, our data suggest that nucleosomes regulate PHO5 transcription.

Gene Expression Regulation↗
PubMed1987

Histone H2B repression causes cell-cycle-specific arrest in yeast: effects on chromosomal segregation, replication, and transcription.

To determine which cellular processes are dependent on histones, we blocked histone H2B mRNA synthesis in asynchronously growing yeast after fusing the H2B gene to a repressible GAL10 promoter. Chromosomal segregation, replication, and transcription were then examined. We found that the cells arrested in mitosis, with a cell division cycle (cdc) phenotype. Chromatin structure and nuclear segregation were disrupted. A full round of DNA replication took place after the repression of histone H2B mRNA synthesis. Active transcription and the induction of new transcripts also continued in the arrested cells.

Cell Cycle↗
PubMed1986

Yeast histone H2A and H2B amino termini have interchangeable functions.

The N-terminal ends of histones H2B and H2A have very different sequences and rates of evolution. However, they both extend from the nucleosome core and are positively charged. Short sequences at the C termini of both proteins also differ from each other and appear to be hydrophilic. Deletions at the N and C termini of yeast histones H2B2 and H2A1 do not obviously affect the cell's viability under normal growth conditions. However, deletions at the N termini of both H2B and H2A in the same cell are lethal or result in greatly reduced viability. Even switching portions of the N termini between H2B and H2A to create two chimeric histone proteins within the same cell has no obvious effect on viability. This supports the argument that the N-terminal end of one protein complements the function of the other.

Amino Acid Sequence↗
PubMed1986

Adenovirus stimulation of transcription by RNA polymerase III: evidence for an E1A-dependent increase in transcription factor IIIC concentration.

Human cells expressing adenovirus E1A proteins transcribe transfected tRNA and adenovirus VAI genes at greater than 10-fold higher levels than uninfected HeLa cells. Here we show that the increased transcription observed in vivo is reflected in the in vitro transcriptional activity of cell extracts. Depletion of E1A protein from these extracts by immunoprecipitation with a monoclonal antibody did not diminish the activity, suggesting that E1A proteins do not stimulate transcription directly. Fractionation of the extracts by chromatography on phosphocellulose suggests that the higher activity of extracts of adenovirus-infected cells was due to increased activity of the transcription factor (TF) which is the limiting component required for specific initiation of tRNA and VAI transcription in extracts of uninfected HeLa cells, i.e. TFIIIC. Template commitment titrations further suggest that the increased TFIIIC activity was due to an increase in the concentration of active TFIIIC. On the basis of these results and recent genetic analyses of early adenovirus promoters, we suggest that E1A proteins stimulate transcription of adenovirus genes indirectly by increasing the effective in vivo concentration of the limiting cellular transcription factors required for their transcription.

Adenovirus Early Proteins↗
PubMed1980

Crossed aphasia in a right-handed bilingual Chinese man: a second case.

A persistent nonfluent aphasia following a right cerebral infarction developed in a 74-year-old right-handed Chinese man. Computerized axial tomography localized the lesion in the right frontal lobe. This case is similar to a previously reported one, also with crossed aphasia in a bilingual and dextral Chinese person. Despite these two independent observations and in spite of the fact that unusual cerebral organization might be related to early learning of a nonalphabetic (ideographic) language, preliminary sampling has failed to demonstrate a greater incidence of crossed aphasia in two separate Chinese populations.

Aged↗
PubMedDate not supplied

High-resolution structural studies of the retinal--Glu113 interaction in rhodopsin.

The key to understanding the reaction mechanism of rhodopsin lies in determining the structure of the retinal binding site and in defining the charge interactions between Glu113 and the retinal protonated Schiff base chromophore. We have been using 13C-NMR chemical shift data to determine the location of the Glu113 carboxyl side chain in relation to the retinal. The NMR data constrain one of the carboxylate oxygens of Glu113 to be ca. 3 A from the C12 position of the retinal with the second oxygen oriented away from the conjugated chain. A water molecule forming a hydrogen bond with the Schiff base is incorporated into the model to account for the high C = N stretching frequency [Han et al., Biophys. J., 65 (1993) 899]. In this study, we have refined the counterion position and have shown that it can reproduce the observed chemical shift data as well as the red-shifted absorption maximum of rhodopsin. Furthermore, the retinal binding site geometry derived from the NMR constraints can be readily incorporated into a recent structural model of the apoprotein.

Amino Acid Sequence↗