Selection versus Screening

Identifying interesting variants in large combinatorial libraries can be accomplished either by assaying all members individually or by applying conditions that allow only variants of interest to appear. Unfortunately, the terms screening for the former strategy and selection for the latter are often confounded in the literature. The basic difference between these approaches is illustrated schematically in Fig. 3.1. 

The bee can distinguish nectar-producing from non-producing flowers on the basis of their distinct phenotypes. (C) Selection. The bee only encounters the desired flowers because nectar production was a prerequisite for plant growth in this case. 

In contrast, selection strategies exploit conditions favoring exclusive survival of desired variants, thereby mimicking the culling process associated with true Darwinian evolution. Panel C of Fig. 3.1 shows how the bee easily encounters nectar-producing flowers under selective conditions that eliminate the flowers that do not produce nectar. 

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More recently, screening efforts at Novartis have identified a hydroxamic acid containing a benzothiazinone ring system (32) [108]. This inhibitor is very potent versus S. aureus Ni -PDF (<5nM) and displays good selectivity versus matrix metalloprotease-2 (MMP-2) and MMP-13. Unfortunately (32), and all other analogues prepared, such as carbon isosteres (33), sulfones (34), N-substituted analogues (35) and N-formyl-N-hydroxylamines (36), lacked appreciable antibacterial activity in spite of their potent enzyme inhibitory activity. Further studies performed by Novartis suggest that these molecules are unable to penetrate the outer cell membrane of E. coli, and may bind to the cell membrane of S. aureus [108]. 

In principle, the screening concepts introduced here could be applied to other polymerases, and other activity. As yet, however, the contexts of low polymerase fidelity or of tolerance versus non-natural substrates have not been sufficiently targeted. In the course of our studies, we tackled these problems and presented solutions for both, selecting, or screening, polymerase libraries. Thereby, we detected an error-prone polymerase variant and polymerase activity in the sole presence of sterically demanding substrates. 

Grtinert SC, et al. Propionic acidemia neonatal versus selective metabolic screening. J Inherit Metab Dis. 2012 35(l) 41-9. 

When you have a clipping circuit, as in the last example, a transfer curve may be desired. This can easily be done by changing the x-axis. A transfer curve usually plots VQ versus Vjn, so we need to change the x-axis from Time to VQ/IN). We must first delete the trace VflfINl from the plot above. Click the LEFT mouse button on the text V/VIN). The text will appear in red, indicating that it has been selected. Once the trace is selected, press the DELETE key to delete the trace. You can delete the other text items and lines on the screen using the same method. Click the LEFT mouse button on an item to select it and then press the DELETE key to delete it. You should have a screen with a single trace ... 

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