CH391L/S12/In vitro Selection

Overview of in vitro selection

Increased Representation

Once diversity is created, the selection must must allow function variants to become a larger percentage of the pool. This step is often the most difficult to design.

Affinity

The easiest function to enrich for is affinity for a ligand. To select for binders (aptamers for nucleic acids; antibodies and others for proteins,) one exposes the pool of potential binders to a fixed ligand. The best binders affix to the ligand while weaker binders are washed away. Those that remain can be amplified for further round of selection. For nucleic acids, the binder itself can be subject to amplification, as it is both the information-carrying and function-carrying molecule. For protein binders, the scheme must include linking of the information-carrying nucleic acid to the function-carrying protein. Some examples of this linking are phage display, cell-surface display, and ribosome display.

Protection

The first in vitro evolved protein functions involved modification of the nucleic acid species that encoded it (See "DNA Modifying Proteins" below). Perhaps the first such function was protection of the DNA template. In Tawfik and Griffiths (1998) The template encodes HaeIII methyl transferase, which then methylates recognition sequences in the gene. The methylated genes are then protected from digestion by HaeIII endonuclease. Undigested templates are then amplified before the next round.

History

Nucleic Acids

Aptamers

Ellington 1990

Ribozymes

Bartel 1993

Proteins

Antibodies

Accepting suggestions

DNA Modifying Proteins

Tawfik 1998

Other Enzymatic Functions

Griffiths 2003

Self-Replication

Ghadessy 2001