The shorter the better: Reducing fixed primer regions of oligonucleotide libraries for aptamer selection

Oligonucleotide aptamers are highly structured DNA or RNA molecules, or modified versions thereof, that can bind to targets with specific affinities comparable to antibodies. They are identified through an in vitro selection process termed SELEX (Systematic Evolution of Ligands by EXponential enrichment) to recognize a wide variety of targets, from small molecules to proteins, and from cultured cells to whole organisms. Aptamers possess a number of desirable properties, such as ease of synthesis, stability, robustness, and lack of immunogenicity. Standard SELEX libraries require two primers, one on each side of a central random domain, to amplify the target-bound sequences via PCR or RT-PCR. However, these primer sequences cause non-specific binding by their nature, and have been reported to lead to large numbers of false-positive binding sequences, or to interfere with binding of sequences within the random regions. This review is focused on methods which have been developed to eliminate fixed primer interference during the SELEX process.