Gene, leaky expression

The use of a gene such as p53 to induce apoptosis of the cell is another feature aimed at avoiding unwanted harm to uninfected cells. This is because if leaky expression of p53 were able to exist in the absence of Rev, this physiological protein would be expressed at low levels in normal cells that do not compromise the cell life, unless sustained high levels of expression were to be achieved. In fact, it might have a protective role against cancer for the widely known tumour suppression activity of this gene. 

In 1991, several factors were demonstrated to affect the efficiency of relE controlled killing of E. colt when under lac promoter control (Knudsen Karlstrom, 1991)- Cells escaped suicide primarily because of the mutation rate and the leakiness of repression during normal growth. When relF was under lac JV5 promoter/operator control, the inactivation of suicide function through spontaneous mutation occurred at a frequency of <5 x 10 9. Knudsen has further theorized that if the number of suicide minus (mutant) bacteria in a culture can be kept at zero before induction, all cells will die. This can be achieved in two ways provide the suicide function in duplicate, and control the suicide gene expression so stringently, with a chromosomally located laclq gene, that a basal level of the suicide function to which cells can adapt will not be present (Knudsen et al., 1995). 

Figure 2.5 Conceptual examples of benefits and limitations of tightly regulated and leaky promoters. Therapeutic ranges for expressed genes are depicted by dotted lines. The left panel describes a situation in which the tightly-controlled promoter achieves regulated expression of therapeutic levels of gene product. The right panel illustrates an example where the leaky promoter is optional.

Undeniably, not all targets are suited for HTS in recombinant microbes. In our own experience, we found that we were unable to complement a PFK strain of S. cerevisiae with the cDNA encoding H. contortus PFK, despite the fact that it was functionally expressed in E. coli (unpublished observations). Conversely, we initially attempted to construct an ODC screen using a strain of E. coli that was unable to synthesize poly amines [40], However, this strain required mutations in multiple genes in order to achieve the polyamine- phenotype, and proved to be too leaky for routine screening [40], The ODC- strain of S. cerevisiae was much better suited for HTS. 

Despite the importance of alternative splicing for the diversity of the proteome and regulation of gene expression two aspects of protein-coding mRNAs— alternative translation initiation and short peptides—have not yet been touched upon. Alternative translation initiation can generate from one transcript two or more protein isoforms. In the case of leaky scanning the ribosome bypasses the... 

---