Selection stringency

The limited space for this review precludes a proper review of the extensive literature on this subject. I will confine this section to results concerning setting selection stringency and mutation rate, as these issues are critical to any laboratory implementation of search. There are four relevant bodies of literature adaptive walk studies, theoretical immune system studies, formal spin glass studies, and theoretical genetic algorithm studies. Much of the adaptive walk literature has already been discussed above. 

The degree of selection can be adjusted as needed by the use of different vectors or promoters to alter the expression level of the enzyme within the cell. Alternatively, the stringency of selection can be increased by channeling the substrate away by adding a competing pathway [42]. 

Kleeb, A.C., Edalat, M.H., Gamper, M. et al. (2007) Metabohc engineering of a genetic selection system with tunable stringency. Proceedings of the National Academy of Sciences of the United States of America, 104, 13907—13912. 

The first two rounds are performed under low stringency conditions to enhance RNA-protein binding and to avoid early depletion of sequences present in the SELEX RNA pool. For SELEX cycles 1-3 a nitrocellulose-filter binding assay is used to separate receptor-bound from free aptamers. Beginning from SELEX cycle 4, the nitrocellulose-filter binding and a gel-shifr selection step are employed as two consecutive selection processes (see Note 2). 

Weit They had different methods for selecting waveform it is not just a matter of stringency. 

To prevent the selection of aptamers with low affinity to the target molecule, the target concentration has to be decreased and the washing procedure has to be done with increased stringency. This enhances the discrimination between aptamers with high affinity and aptamers with lower affinity [6]. 

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