An immunological assay for chloramphenicol acetyltransferase

Cellular expression of an HMGCR promoter-CAT fusion gene in transgenic mouse brain: evidence for a developmental regulation in oligodendrocytes

The HMGCR gene encodes the 3-hydroxy-3-methylglutaryl coenzyme A reductase, which is the key enzyme for cholesterol synthesis. Mice transgenic for the prokaryotic chloramphenicol acetyl transferase (CAT) reporter gene fused with a 5' Bam H1 fragment including the promoter sequence for murine HMGCR gene have been obtained. Homozygote transgenic mice were derived from a particular line selected for similar regulation of endogenous HMGCR and the transgene expression by nutritional conditions in different tissue. In addition, high expression of the transgene was evidenced in the brain. Cellular expression of the CAT gene in the central nervous system (CNS) was investigated by immunohistochemistry (IHC). This study was performed on frozen sections of the developing and adult brain, using a rabbit anti-CAT antiserum especially raised for that purpose. CAT expression was observed in some rare individuals in different neural cell types including Purkinje cells and astrocytes. But the most outstanding observation was the high level of CAT expression correlated with differentiated pattern of oligodendrocyte (Ol) distribution observed in white-matter tracts. Double and triple labeling for CAT and stage-specific antigens were performed on transgenic Ol-enriched preparations and cultures. This study showed a normal sequence of differentiation in the transgenic oligodendroglial cell lineage and demonstrated a strict correlation between late differentiation and activation of the CAT gene in these cells: CAT expression started in transgenic Ols between galactocerebroside (GC)-positive and myelin basic protein (MBP)-positive stages and was detected in MBP-positive cells during the myelination period. After myelination, the number of CAT-positive Ols decreased in the adult brain. These observations demonstrate a developmental regulation of the CAT transgene in Ols during myelination in CNS and reinforce the hypothesis of endogenous synthesis as major source of cholesterol during myelination.

Quantitation of chloramphenicol acetyl transferase in transgenic tobacco plants by ELISA and correlation with gene copy number

A monoclonal antibody to chloramphenicol acetyl transferase (CAT) was used in an indirect competitive enzyme immunoassay (ELISA) for the quantitation of CAT in leaf extracts of eighteen transgenic tobacco plants containing the CAT gene fused to the cauliflower mosaic virus 35S promoter. The ELISA could be used to quantify CAT when present in extracts at 20 ng/ml. Enzymatic activity and electrophoretic mobility of CAT in these extracts was not different from CAT from Escherichia coli. Concentrations of CAT in these transgenic plants ranged from 79 to 732 ng CAT/mg protein. The average coefficient of variation among three replicate samples was 15%. All plants were sampled on two separate occasions. The CAT concentrations often varied between the two sampling dates. We determined the CAT gene copy number and the number of independently segregating loci in each plant by Southern blot analysis and progeny testing. We found no significant differences in CAT expression among all ten plants with a single CAT gene. We also found a significant correlation between CAT gene copy number and the level of CAT expressed in each plant, although plants with one gene copy sometimes had more CAT than plants with more than one gene copy. In this population, therefore, gene copy number contributed more to the variation in CAT expression than did position effects.

Variation in enzymatic transient gene expression assays

We examined causes for high variability in data from enzymatic transient gene expression assays. Our results strongly suggest that variation in transfection efficiency is the major cause of data variation and can seriously compromise valid interpretation of data. We compared averaging data from multiple transfections and cotransfection of a second reporter gene as methods for correcting for variation in transfection efficiency. We found that transfection efficiency can be so highly variable that neither method necessarily overcomes the resulting bias in data. Depending upon the degree in variation in transfection efficiency, a combination of the two methods may be advisable. The need to normalize data for transfection efficiency is dependent upon the difference in strengths of promoters being tested and the relative variability of the transfection method used. We also show that the level of reporter gene expression between transfection experiments performed on different days can vary by more than 10-fold.

Novel eukaryotic expression vectors which permit single-stranded replication in Escherichia coli and in vitro translational analysis of cloned genes

The ability to express cloned genes transiently is an important technique in the study of eukaryotic gene expression. Numerous useful expression vectors have been constructed for this purpose although many of them share several common drawbacks. In this paper I describe the construction and characterization of novel expression vectors pSVKII and pSVKIII which have 13 and 8 unique restriction sites, respectively, suitable for cloning genes. These vectors have phage M13 ori, which permits them to produce and package ss DNA molecules in Escherichia coli host, thus facilitating the sequencing and site directed mutagenesis of a cloned gene. Expression vector pSVKIII has a T7 phage promoter located between the SV40 promoter and the multiple cloning sites, which permits efficient transcription and in vitro translation of the cloned gene prior to in vivo studies.

A simple phase-extraction assay for chloramphenicol acyltransferase activity

A simple and convenient phase extraction assay for chloramphenicol (Cm) acetyltransferase (CAT) activity has been developed, based on the enzymatic butyrylation of radiolabelled Cm. The assay is linear over two to three orders of magnitude of enzyme concentration, is highly sensitive, and substantially less expensive than all presently available alternatives. Methods for convenient CAT assay, adapted for mammalian cells, plant protoplasts, and mammalian cell culture supernatants, are described. These methods should also simplify measurement of CAT activity in other organisms, such as yeast and bacteria. In addition, a simple pre-extraction procedure is presented for purifying radiolabelled Cm which allows a 25-fold increase in sensitivity using tritiated substrates.