7 Main Characteristics of a Good Host Cell

The following points highlight the seven main characteristics of a good host cell.

1. Can allow the easy entry of the recombi­nant DNA easily into the cell. This pro­cess is poorly understood. Some mutations appear to enhance the efficiency of DNA uptake itself. These include the deoR mu­tation in E. coli, which seems to assist par­ticularly in the uptake of larger DNA mole­cules.

2. Should not destroy the recombinant DNA as a foreign DNA and degrade it. The main feature determining transformation effi­ciency is the presence or absence of endoge­nous DNA-degrading systems. Many hosts used for cloning are derived from E. coli strain K, which contains the K restriction modification system encoded by the hsdRMS locus.

The hsdR gene encodes an endonuclease that cleaves DNA containing the sequence -AACNNNNNNGTGC−, unless the second of the two adenine resi­dues and the adenine residue on the other strand opposite to the thymine are methy­lated.

Many hosts, therefore, have an hsdR mutation (or a larger deletion) to avoid cleavage of incoming unprotected DNA. The hsdM gene encodes the methylase that protects against degradation so passaging of DNA through an hsdR⁻ M⁺ strain can be used to allow methylation if it is subse­quently necessary to propagate in an hsdR⁺ strain.

3. Can stably maintain the recombinant DNA. Once a recombinant DNA has en­tered the cell, it is still not guaranteed to replicate indefinitely and stably even if it has a suitable origin of replication. Rear­rangement of the recombinant may occur, and the most frequent manifestation of this is partial deletion (i.e., loss of part of the molecule).

This usually occurs by re­combination across directly repeated se­quences. Due to this the chosen host cell must lack the genes producing enzymes for a endogenous recombination process. For example, we take E. Coli mutants for rec A and rec F genes.

4. The transformed host must not indepen­dently sustain outside the laboratory. As with vectors, it is necessary to take pre­cautions to ensure that strains carrying recombinant plasmids are unlikely to es­cape and propagate outside the laboratory.

For this reason, the preferred strains usu­ally carry mutations that reduce their via­bility in the wild. These are often muta­tions conferring auxotrophy (i.e. the re­quirement for a particular metabolite to be supplied in the medium, resulting from an inability to synthesize the metabolite).

5. Should be easy to maintain and handle.

6. Should be available as a wide variety of genetically defined strains.

7. Should accept a range of vectors.