A common method for transformation of plasmids into E.coli is the use of chemically competent cells. Although competent cells can be prepared in the laboratory, greater efficiency, reproducibility, and convenience are achieved using Novagen® prepared competent cells. Every Novagen® competent cell strain is verified for phenotype and purity, and is guaranteed for transformation efficiency.

DE3 indicates that the host is a lysogen of λDE3, and therefore carries a chromosomal copy of the T7 RNA polymerase gene under control of the lacUV5 promoter. Such strains are suitable for production of protein from target genes cloned in pET vectors by induction with IPTG.

pLysS strains express T7 lysozyme, which further suppresses basal expression of T7 RNA polymerase prior to induction, thus stabilising pET recombinants encoding target proteins that affect cell growth and viability.

The standard volume per tube is 0,2 ml, which provides enough cells for 10 transformations. All kits are supplied with SOC medium and test plasmid. Several strains are also available as Singles™ Competent Cells, which are provided in single-use, 50 μl volumes for extra convenience and efficiency.

Rosetta™ and Rosetta 2™ host strains are BL21 derivatives designed to enhance the expression of eukaryotic proteins that contain codons rarely used in E. coli. These strains supply tRNAs for AGG, AGA, AUA, CUA, CCC, GGA codons on a compatible chloramphenicol-resistant plasmid. Thus the Rosetta™ strains provide for 'universal' translation which is otherwise limited by the codon usage of E. coli. The tRNA genes are driven by their native promoters. In Rosetta(DE3)pLysS, the rare tRNA genes are present on the same plasmids that carries the T7 lysozyme gene.

Rosetta-gami™ B strains combines the features of BL21 (and its Tuner™ derivative), Origami™ and Rosetta™ to enhance both the expression of eukaryotic proteins and the formation of target protein disulfide bonds in the bacterial cytoplasm. These strains are compatible with ampicillin- or spectinomycin-resistant vectors.