Identification and characterization of mutant clones with enhanced propagation rates from phage-displayed peptide libraries.

A target-unrelated peptide (TUP) can arise in phage display selection experiments as a result of a propagation advantage exhibited by the phage clone displaying the peptide. We previously characterized HAIYPRH, from the M13-based Ph.D.-7 phage display library, as a propagation-related TUP resulting from a G→A mutation in the Shine-Dalgarno sequence of gene II. This mutant was shown to propagate in Escherichia coli at a dramatically faster rate than phage bearing the wild-type Shine-Dalgarno sequence. We now report 27 additional fast-propagating clones displaying 24 different peptides and carrying 14 unique mutations. Most of these mutations are found either in or upstream of the gene II Shine-Dalgarno sequence, but still within the mRNA transcript of gene II. All 27 clones propagate at significantly higher rates than normal library phage, most within experimental error of wild-type M13 propagation, suggesting that mutations arise to compensate for the reduced virulence caused by the insertion of a lacZα cassette proximal to the replication origin of the phage used to construct the library. We also describe an efficient and convenient assay to diagnose propagation-related TUPS among peptide sequences selected by phage display.