Modulation of transcription from chromatin assembled in vitro

Altered exocrine pancreatic function in rats treated with nicotine

The present study investigates the effects of nicotine treatment on exocrine pancreatic function. Adult male, Sprague-Dawley rats received nicotine via a time-release pellet, at a rate of 1.65 micrograms/min for 3 weeks. At the end of the experimental period, it was observed that although nicotine did not affect final body or pancreatic weight, the activities of amylase, trypsin, and chymotrypsin in pancreatic homogenates from nicotine-treated rats were 51, 29, and 35% higher, respectively, than in controls. Levels of immunoreactive cationic trypsin(ogen) were significantly higher in pancreatic homogenates and serum from nicotine-treated rats as compared with controls. In addition, concentrations of mRNA, encoding for pancreatic amylase, were higher in pancreatic homogenates from the nicotine-treated rats than in controls. In dispersed pancreatic acini isolated from nicotine-treated rats, basal secretion of amylase, trypsinogen, and chymotrypsinogen was 50% higher than controls and enzyme release following CCK-8 (100 pM), secretin (1 microM), and carbachol (7.5 microM) stimulation was also significantly higher. These data indicate that nicotine treatment, at levels comparable to those expected in moderate cigarette smokers, increases the content of digestive enzymes in rat pancreas, as well as their basal and secretagogue-induced release.

Pseudomonas aeruginosa cytotoxin stimulates secretion of amylase and protease zymogens with a concomitant decrease of mRNA levels in isolated rat pancreatic acini

The action of Pseudomonas aeruginosa cytotoxin on isolated pancreatic acini was investigated. The release of amylase and serine protease zymogens from the isolated rat pancreatic acini was induced with increasing amounts of cytotoxin in vitro. The stimulated release of amylase reached 30% of total cellular content with 100 micrograms/mL of the purified cytotoxin. The induced release of amylase, trypsinogen, proelastase, and chymotrypsinogen reached the maximum after 75 minutes of incubation while lactate dehydrogenase began to appear after 15 minutes of incubation with a secondary biphasic increase at 75 min of incubation. The concentrations of acinar mRNAs of amylase, trypsinogen, proelastase, and chymotrypsinogen, as measured by dot-blot hybridization with the cloned cDNAs of amylase, trypsinogen I, proelastase II, and chymotrypsinogen B of the rat, decreased with time and were significantly lower than in the untreated acini. It is concluded that cytotoxin stimulates the release of amylase and protease zymogens with a concomitant increase in membrane permeability and a decrease of cellular mRNA levels. The inhibition of gene expression is attributable merely to a generalized toxic effect upon cellular metabolism.

Chromatin structure along the ribosomal DNA of Dictyostelium. Regional differences and changes accompanying cell differentiation

The ribosomal genes of Dictyostelium discoideum are extrachromosomal palindromic DNA molecules situated in the nucleolus. Each molecule comprises ribosomal RNA coding regions and non-transcribed spacer regions. We used both biochemical and electron microscopic approaches to investigate the structure of transcribing and non-transcribing chromatin. Nucleoli from exponentially growing cells were digested with micrococcal nuclease, and the resulting DNA fragments were separated by gel electrophoresis and transferred to DBM paper. They were hybridized with cloned EcoRI fragments derived from different parts of the ribosomal gene. Probes of the coding region showed a smear, while probes of the non-transcribed regions gave pronounced banding patterns more complex than typical nucleosome repeats, but not due solely to sequence-specific cutting by micrococcal nuclease. The DNA of the coding region was digested more quickly than that of the non-transcribed ones. When nucleoli were digested with restriction enzymes, sites within the coding region were accessible and sites in the non-transcribed region were protected. The structure of ribosomal chromatin in differentiating cells, in which the rate of ribosomal RNA synthesis is reduced, was examined using essentially the same methods. The coding region, probed by hybridization to micrococcal digests, then showed a typical DNA repeat pattern indicating that this region had become condensed into nucleosomes, and its accessibility to restriction enzymes was very much reduced. On electron micrographs of lysed nucleoli from exponentially growing cells, two types of chromatin were observed, one with a beaded nucleosomal appearance, the other with putative RNA polymerase molecules attached to fibres indistinguishable from free DNA adsorbed to the same grid. The combined results suggest that whereas regions that are not transcribed are packaged with proteins that protect them from nuclease digestion, actively transcribing ribosomal genes are associated with few macromolecular constituents apart from those required for transcription and its regulation.

Sequence analysis of the cloned mRNA coding for glyceraldehyde-3-phosphate dehydrogenase from chicken heart muscle

Using a cloned cDNA (pGAP30) the nucleotide sequence for chicken glyceraldehyde-3-phosphate dehydrogenase mRNA has been determined. The cDNA insert contains 1051 nucleotides representing the amino acid coding sequence, with the exception of 49 NH2-terminal amino acids, and includes the entire 3'-noncoding region. Sequence information on the missing 5' terminus of the mRNA, not represented in the clone pGAP30, was obtained by extension of the cDNA using an 85-nucleotide-long internal fragment as a primer. Thus the sequence of 310 amino acids of chicken glyceraldehyde-3-phosphate dehydrogenase representing 93% of the complete primary structure could be derived. The coding portion exhibits non-random utilization of synonymous codons with a strong bias for codons with G or C at the third position. The non-coding region contains several octanucleotides which are repeated and shows a potentially stable stem-and-loop structure located towards the end of the mRNA. Hypothetical functional implications of the putative secondary structure are discussed.