Rosetta™ host strains are BL21 derivatives designed to enhance the expression of eukaryotic proteins that contain codons rarely used in E. coli.

• tRNA genes are driven by their native promoters
• Standard for 20 or 50 reactions
• Guaranteed efficiency: >2×10⁶ cfu/µg for host strains

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.

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 lac UV5 promoter. Such strains are suitable for production of protein from target genes cloned in pET vectors by induction with IPTG. In Rosetta(DE3)pLysS, the rare tRNA genes are present on the same plasmids that carries the T7 lysozyme gene.

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