No abstract

Oxytetracycfine-resistant (OTcr) and tetracycline-resistant (Tc') Aeromonas hydrophila were isolated commonly from catfish intestinal contents and the water and sediment of catfish culture ponds, but less frequently in market catfish. Isolates demonstrated two resistance phenotypes, Tcr OTcr and Tcs OTcr, when plated directly on to MacConkey agar containing 30 Fig of tetracycline or oxytetracycline per ml. Tcs OTcr isolates expressed Tcr after induction by 1 h of growth in tryptic soy broth containing 1 p.g of tetracycline per ml, Over 90% of the resistant aeromonads hybridized with DNA probes for class A or class E Tcr determinants; class E was twice as prevalent as class A. The distribution of class A and E Tcr determinants varied with the Tcr phenotypes. Prior to induction, 86% of isolates with class A determinants were Tcr as well as OTcr, while only

Crude wheat germ nuclear extracts contain many inhibitors of transcription which need to be removed before an active system can be developed. Using ion exchange column chromatography to resolve RNA polymerase II transcription components we can identify at least four fractions required for transcription by their ability to interact with, or substitute for, particular HeLa fractions. Inhibitors can be removed by a second or third chromatographic process applied to each fraction. Two plant fractions can each effectively replace the corresponding fraction in a HeLa transcription system, and the wheat fractions can work together and replace two HeLa fractions. These plant factors chromatograph identically to HeLa factors on ion exchange columns. The third fraction does not fully substitute for the corresponding HeLa fraction, but can complement this HeLa fraction when both are added at half-optimal levels. An in vitro plant system consisting of four plant chromatographic fractions will selectively transcribe a gene, but only at very low efficiency. The apparent block to greater efficiency is in elongation of the RNA past the 20-30n size.

Crude plant cell protein extracts prepared from wheat germ are inactive for in vitro transcription by RNA polymerase II. These extracts do, however, have correct initiation of transcription by RNA polymerase II. Initiation is monitored by measuring the formation of transcription complexes in vitro. A nuclear extract produces more initiation events than a whole cell extract or a cytosol extract. Some factors necessary for initiation can be separated from other proteins, including inhibitors, by ion exchange column chromatography. One specific fraction is sufficient for the formation of transcription complexes and several other fractions may be stimulatory or accessory factors.