You may want to quickly review transcription and translation before jumping into the regulation of transcription and translation.
Gene expression can be regulated at several different levels, for example:
Operon fusions place the transcription of a reporter gene under the control of an upstream gene. Because the reporter gene has its own efficient translation start sites and because the reporter gene product is not posttranslationally regulated, the amount of the reporter gene product is proportional to the transcription of the upstream gene. Translation of the mutated gene stops before the reporter gene, and translation reinitiates at the translational start sites in front of the reporter gene. Hence, two gene products result -- the truncated protein and the reporter protein. The transposon derivative MudJ forms operon fusions.
Gene fusions place both the transcription and translation of a reporter gene under the control of an upstream gene. When inserted in the correct reading frame, there is no stop codon and not translational start sites in front of the reporter gene, so translation continues from the disrupted gene product directly into the reporter gene product. Hence, one gene products results -- a hybrid protein with the N-terminus of the truncated protein and the C-terminus of the reporter protein. The transposon derivative MudK forms gene fusions.
By comparing the effect of regulatory mutations on gene expression in a strain with an operon fusion in a gene with another strain with a gene fusion located at the same site within the gene, it is possible to determine if the effect of the mutations is at a transcriptional or translational level. If a mutation affects transcription of the gene, then it will increase or decrease expression of the reporter gene in both the operon and gene fusions to a similar extent. In contrast, if the mutation affects translation of the gene, then it will increase or decrease expression of the reporter gene in both the gene fusion but not the operon fusion. A simplified example is shown below. In this example, the expression of the lacZ gene from an operon and a gene fusion in gene X is compared in four different mutant strains.
Caveats: Operon and gene fusions can provide useful insights into genetic regulation, and enable genetic approaches for isolating mutations that affect the gene products and sites that mediate genetic regulation. However, both gene and operon fusions can be affected by changes in the stability of mRNA, either because the fusion may displace a mRNA sequence involved in stabilizing or destabilizing the transcript, or because the join point between the upstream transcript and the fusion affects the stability of the mRNA. Some good examples of potential problems are described in the reference by Pezzi et al. (2001).
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Last modified September 22, 2001