For routine subcloning, nearly every manufacturer has a repertoire of standard laboratory strains. Many companies have expanded their range of products with special-function host strains that overcome many of the critical problems encountered during cloning and maintaining non-E. coli sequences in bacteria.
Stratagene has a sizable entry in the special-use bacterial strain segment. Among some of the high-efficiency strains are those for maximizing or exploiting aspects of protein expression. The TOPP, ABLE C, and ABLE K E. coli strains assist in expressing toxic or unstable proteins by limiting plasmid copy numbers. The TKB1 and TKX1 strains are engineered to have an inducible elk tyrosine kinase gene controlled by the trp promoter. When expressed, this gene phosphorylates proteins in the host strain. This feature is useful for affinity screening of libraries, for purification of SH2 domain-containing proteins, or for other strategies in which phosphorylated residues have a selective advantage.
Highly repetitive sequences are often very unstable in bacteria and can lead to sequence rearrangements and deletions. These events ultimately generate sequence alterations and abnormalities that complicate sequence analysis. Life Technologies' Max-Efficiency STBL2 Competent Cells are uniquely engineered host strain intended to stabilize direct repeat and retroviral sequences found in viruses such as HIV and SIV.
Novagen provides a broad assortment of host strains for protein expression. Many strains are specially engineered for use with pET expression-based vectors. The AD494 host strains, for example, are thioredoxin reductase (trxB) mutants that enable disulfide bond formation. These mutants provide the potential for the formation of properly folded, active recombinant protein. The B834 derivatives of the BL21 host strain B834 are methionine-deficient strains, which allow for high specific activity labeling of target proteins, while the BLR host contains a mutation that may help stabilize target sequences containing repetitive sequences.
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