▪ Abstract The prostaglandin endoperoxide H synthases-1 and 2 (PGHS-1 and PGHS-2; also cyclooxygenases-1 and 2, COX-1 and COX-2) catalyze the committed step in prostaglandin synthesis. PGHS-1 and 2 are of particular interest because they are the major targets of nonsteroidal anti-inflammatory drugs (NSAIDs) including aspirin, ibuprofen, and the new COX-2 inhibitors. Inhibition of the PGHSs with NSAIDs acutely reduces inflammation, pain, and fever, and long-term use of these drugs reduces fatal thrombotic events, as well as the development of colon cancer and Alzheimer's disease. In this review, we examine how the structures of these enzymes relate mechanistically to cyclooxygenase and peroxidase catalysis, and how differences in the structure of PGHS-2 confer on this isozyme differential sensitivity to COX-2 inhibitors. We further examine the evidence for independent signaling by PGHS-1 and PGHS-2, and the complex mechanisms for regulation of PGHS-2 gene expression.

Cyclooxygenases: Structural, cellular, and molecular biology

In this chapter we have provided instructions for transforming yeast by a number of variations of the LiAc/SS-DNA/PEG method for a number of different applications. The rapid transformation protocol is used when small numbers of transformants are required. The high efficiency transformation protocol is used to generate large numbers of transformants or to deliver DNA constructs or oligonucleotides into the yeast cell. The large-scale transformation protocol is primarily applicable to the analysis of complex plasmid DNA libraries, such as those required for the yeast two-hybrid system. The microtiter plate versions of the rapid and high efficiency transformation protocols can be applied to high-throughput screening technologies.

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Transformation of yeast by lithium acetate/single-stranded carrier DNA/polyethylene glycol method

Here we describe a high-efficiency version of the lithium acetate/single-stranded carrier DNA/PEG method of transformation of Saccharomyces cerevisiae. This method currently gives the highest efficiency and yield of transformants, although a faster protocol is available for small number of transformations. The procedure takes up to 1.5 h, depending on the length of heat shock, once the yeast culture has been grown. This method is useful for most transformation requirements.

High-efficiency yeast transformation using the LiAc/SS carrier DNA/PEG method

Disruption-deletion cassettes are powerful tools used to study gene function in many organisms, including Saccharomyces cerevisiae. Perhaps the most widely useful of these are the heterologous dominant drug resistance cassettes, which use antibiotic resistance genes from bacteria and fungi as selectable markers. We have created three new dominant drug resistance cassettes by replacing the kanamycin resistance (kan(r)) open reading frame from the kanMX3 and kanMX4 disruption-deletion cassettes (Wach et al., 1994) with open reading frames conferring resistance to the antibiotics hygromycin B (hph), nourseothricin (nat) and bialaphos (pat). The new cassettes, pAG25 (natMX4), pAG29 (patMX4), pAG31 (patMX3), pAG32 (hphMX4), pAG34 (hphMX3) and pAG35 (natMX3), are cloned into pFA6, and so are in all other respects identical to pFA6-kanMX3 and pFA6-kanMX4. Most tools and techniques used with the kanMX plasmids can also be used with the hph, nat and patMX containing plasmids. These new heterologous dominant drug resistance cassettes have unique antibiotic resistance phenotypes and do not affect growth when inserted into the ho locus. These attributes make the cassettes ideally suited for creating S. cerevisiae strains with multiple mutations within a single strain.

Three new dominant drug resistance cassettes for gene disruption in Saccharomyces cerevisiae.

Regulatory elements that control tetracycline resistance in Escherichia coli were previously converted into highly specific transcription regulation systems that function in a wide variety of eukaryotic cells. One tetracycline repressor (TetR) mutant gave rise to rtTA, a tetracycline-controlled transactivator that requires doxycycline (Dox) for binding to tet operators and thus for the activation of Ptet promoters. Despite the intriguing properties of rtTA, its use was limited, particularly in transgenic animals, because of its relatively inefficient inducibility by doxycycline in some organs, its instability, and its residual affinity to tetO in absence of Dox, leading to elevated background activities of the target promoter. To remove these limitations, we have mutagenized tTA DNA and selected in Saccharomyces cerevisiae for rtTA mutants with reduced basal activity and increased Dox sensitivity. Five new rtTAs were identified, of which two have greatly improved properties. The most promising new transactivator, rtTA2S-M2, functions at a 10-fold lower Dox concentration than rtTA, is more stable in eukaryotic cells, and causes no background expression in the absence of Dox. The coding sequences of the new reverse TetR mutants fused to minimal activation domains were optimized for expression in human cells and synthesized. The resulting transactivators allow stringent regulation of target genes over a range of 4 to 5 orders of magnitude in stably transfected HeLa cells. These rtTA versions combine tightness of expression control with a broad regulatory range, as previously shown for the widely applied tTA.

https://www.pnas.org/content/pnas/97/14/7963.full.pdf

Exploring the sequence space for tetracycline-dependent transcriptional activators: Novel mutations yield expanded range and sensitivity

An improved lithium acetate (LiAc)/single-stranded DNA (SS-DNA)/polyethylene glycol (PEG) protocol which yields >1 × 106 transformants/μg plasmid DNA and the original protocol described by Schiestl and Gietz (1989) were used to investigate aspects of the mechanism of LiAc/SS-DNA/PEG transformation. The highest transformation efficiency was observed when 1 × 108 cells were transformed with 100 ng plasmid DNA in the presence of 50 μg SS carrier DNA. The yield of transformants increased linearly up to 5 μg plasmid per transformation. A 20-min heat shock at 42°C was necessary for maximal yields. PEG was found to deposit both carrier DNA and plasmid DNA onto cells. SS carrier DNA bound more effectively to the cells and caused tighter binding of 32P-labelled plasmid DNA than did double-stranded (DS) carrier. The LiAc/SS-DNA/PEG transformation method did not result in cell fusion. DS carrier DNA competed with DS vector DNA in the transformation reaction. SS plasmid DNA transformed cells poorly in combination with both SS and DS carrier DNA. The LiAc/SS-DNA/PEG method was shown to be more effective than other treatments known to make cells transformable. A model for the mechanism of transformation by the LiAc/SS-DNA/PEG method is discussed.

R D Gietz

Papers

Studies on the transformation of intact yeast cells by the LiAc/SS‐DNA/PEG procedure