No abstract

The mechanisms by which viral regulatory proteins activate the cellular transcription apparatus without binding to specific DNA elements are not fully understood. Several lines of evidence suggest that activation by one such regulatory protein, herpes simplex virus ICP4, could be mediated, at least in part, by TFIID. To test this model, we replaced the TATA box of the ICP4-responsive viral thymidine kinase gene with functional TATA boxes that displayed different apparent affinities for TATA-box-binding protein as measured by DNase I footprinting. We measured the effects of these TATA boxes on ICP4 induction by constructing ICP4-deficient recombinant viruses containing the different TATA alleles and comparing their expression in cells lacking or expressing ICP4. Overall, ICP4 induced weak TATA boxes (those that displayed low apparent affinity for TATA-box-binding protein and low basal expression) the most (18-to 41-fold) and strong TATA boxes the least (7-to 10-fold). Therefore, ICP4 induction correlated inversely with TATA box strength. Using a reconstituted in vitro transcription assay, we determined that the relative levels of induction by ICP4 of the different TATA alleles were similar to those measured in vivo, suggesting that ICP4 was the only viral protein required for induction. These results fit a model in which ICP4 acts in part to enhance binding of TFIID to the TATA box. We compare and contrast these results with those observed with the viral regulatory proteins adenovirus E1a and simian virus 40 large T antigen and the cellular coactivator PC4.RNA polymerase II (pol II) promoters can roughly be separated into two functional elements: (i) proximal or core elements that consist of the TATA box and the transcriptional start site, and (ii) distal elements such as activator elements and enhancers. Numerous studies investigating the structures and functions of pol II promoters have provided a model in which the regulated expression of eukaryotic genes occurs through the interaction of the core elements which bind the basal transcription factors, including TFIID, TFIIB, TFIIA, TFIIE, TFIIF, TFIIG/J, and TFIIH (6, 10), and the distal elements which bind a wide variety of regulatory proteins such as Sp1 (for reviews, see references 50 and 52). However, some well-studied viral regulatory proteins, including adenovirus E1a, simian virus 40 (SV40) large T antigen, and herpes simplex virus (HSV) ICP4, can activate many genes without sequence-specific binding to DNA and independent of distal regulatory elements (4,23,29,30,38,44,71). This finding indicates that gene induction by activator proteins can involve mechanisms that are independent of specific DNA binding. The mechanisms by which activator proteins induce gene expression in the absence of specific DNA binding are poorly understood. However, some viral regulatory proteins that function independently of specific DNA binding require only a TATA box for induction (28,65,71). Moreover, E1a (36,48) and large T antigen (29) bind directly to either TFIID or more ...

Ribonucleotide reductase activity is required for generating deoxyribonucleotides for DNA replication. Schizosaccharomyces pombe cells lacking ribonucleotide reductase activity arrest during S phase of the cell cycle. In a screen for hydroxyurea-sensitive mutants in S. pombe, we have identified a gene, liz1 ϩ , which when mutated reveals an additional, previously undescribed role for ribonucleotide reductase activity during mitosis. Inactivation of ribonucleotide reductase, by either hydroxyurea or a cdc22-M45 mutation, causes liz1 Ϫ cells in G2 to undergo an aberrant mitosis, resulting in chromosome missegregation and late mitotic arrest. liz1 ϩ encodes a 514-amino acid protein with strong similarity to a family of transmembrane transporters, and localizes to the plasma membrane of the cell. These results reveal an unexpected G2/M function of ribonucleotide reductase and establish that defects in a transmembrane protein can affect cell cycle progression.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. BRILL is collaborating with JSTOR to digitize, preserve and extend access to Muqarnas.

No abstract