DeoP1 and deoP2 promoter fragments from the deo operon of Escherichia coli have been transcriptionally fused to the galactokinase gene. From single‐copy expression of these fusions it is shown that the deoR binding site of both deoP1 and deoP2 are necessary to achieve full repression of the deo operon by the deoR repressor. Repression of the promoters can be achieved either by supplying extra deoR repressor in trans or by introduction of an extra deoR binding site at a position between 224 and 997 bp upstream of the promoter. Furthermore, the deoP2 promoter is shown to be regulated in a cumulative way by both the deoR and the cytR repressors, while deoP1 is only regulated by the deoR repressor. DeoP2 is a strong promoter being 20 times stronger than araPBAD and four times stronger than deoP1.

We review literature on the metabolism of ribo- and deoxyribonucleotides, nucleosides, and nucleobases in Escherichia coli and Salmonella,including biosynthesis, degradation, interconversion, and transport. Emphasis is placed on enzymology and regulation of the pathways, at both the level of gene expression and the control of enzyme activity. The paper begins with an overview of the reactions that form and break the N-glycosyl bond, which binds the nucleobase to the ribosyl moiety in nucleotides and nucleosides, and the enzymes involved in the interconversion of the different phosphorylated states of the nucleotides. Next, the de novo pathways for purine and pyrimidine nucleotide biosynthesis are discussed in detail.Finally, the conversion of nucleosides and nucleobases to nucleotides, i.e.,the salvage reactions, are described. The formation of deoxyribonucleotides is discussed, with emphasis on ribonucleotidereductase and pathways involved in fomation of dUMP. At the end, we discuss transport systems for nucleosides and nucleobases and also pathways for breakdown of the nucleobases.

The deoR gene, which encodes the deor repressor protein in Escherichia coli, was fused to the strong Ptrc promoter in plasmid pKK233‐2. The Ptrc promoter is kept repressed by lacI repressor to prevent cell killing. Induction of the Ptrc–deoR fusion plasmid resulted in the accumulation of 4% of the soluble protein as deoR protein. The deoR repressor protein was purified to 80% purity using conventional techniques; it has a mass of 28.5 kd and appears to exist as an octamer in solution. The deoR repressor is shown by DNase I footprinting to bind to the 16 bp palindromic sequence in the Pribnow box region of the deoP1 promoter. Also, the deoR repressor binds cooperatively in vitro to a DNA template with two deoR binding sites separated by 224 bp in keeping with the conclusion from genetic experiments that more than one operator is required for efficient repression of the deo operon.

Research Question 1: How do media and methods influence the outcomes of transfer, retention and self-efficacy in an immediate posttest? Research Question 2: How do media and methods influence the outcomes of intrinsic motivation, perceived enjoyment and presence in an immediate posttest? Research Question 3: What is the effect of media and methods when students are asked to re-use the simulation with the alternative media on the outcomes of transfer, retention, self-efficacy, presence, perceived enjoyment and intrinsic motivation? Methods Participants The sample consisted of 89 participants (61 females, 28 males, 0 non-binary) from a large European university. The learning intervention was part of a mandatory class required for all firstyear undergraduate biochemistry students. Students ranged in age from 19 to 36 (M = 21.34, SD = 2.18). Research design We employed a 2 × 2 mixed-methods design in a natural classroom setting. In the first part of the study, students were randomly assigned to one of two method conditions (GLS/no-GLS) and one of two initial media conditions (IVR/DVR). In the GLS conditions, students would, in pairs, conduct teaching with one student initially occupying the role as the teacher for five minutes in the audience of the other student, whose role was to pose relevant comments and questions. Then, students

Regulation of transcription initiation by proteins binding at DNA sequences some distance from the promoter region itself seems to be a general phenomenon in both eukaryotes and prokaryotes. Proteins bound to an enhancer site in eukaryotes can turn on a distant gene, whereas efficient repression of some prokaryotic genes such as the gal, ara and deo operons of Escherichia coli, requires the presence of two operator sites, separated by 110, 200 and 600 base pairs (bp) respectively. In the deo operon, which encodes nucleoside catabolizing enzymes, we have shown that efficient and cooperative repression can be obtained when the distance between the two sites ranges from 224 to 997 bp. Here, we report that transcription initiation can be regulated from an operator site placed 1 to 5 kilobases (kb) downstream of the deoP2 promoter (and downstream of the transcribed gene), and present the first experimental data for prokaryotic regulation at distances greater than 1 kb. Our results support the model of DNA loop formation as a common regulatory mechanism explaining both some prokaryotic regulation and the action of eukaryotic enhancers.