The properties of this strain that contribute to its usefulness as a protein expression strain are described below. The genotypes underlying these properties appear in parentheses. Protease Deficient ([lon] ompT): E. coli B strains are ""naturally"" deficient in the lon protease which in K-12 strains serves to degrade misfolded proteins and to prevent some cell cycle-specific proteins from accumulating. The OmpT protease resides at the surface of wild type E. coli in both K-12 and B strains, presumably helping the cells to derive amino acids from their external environment. Cells deficient in both these proteases are much more amenable to the production of proteins from cloned genes. Recovery from DNA Damage (sulA11): E. coli cells can tolerate a substantial amount of chronic DNA damage as long as repair is allowed to proceed. This capacity is compromised if the cells are unable to divide following repair. In lon- cells, SulA, a cell division inhibitor, accumulates and causes cells to become hypersensitive to DNA damage. The sulA mutation introduced into the T7 Express strain allows cells to divide more normally in the absence of Lon protease. Endonuclease I Deficient: (endA) The periplasmic space of wild type E. coli cells contains a nonspecific endonuclease. Extreme care must be taken to avoid degradation of plasmids prepared from these cells. The endA mutation deletes this endonuclease and can significantly improve the quality of plasmid preparations. Restriction Deficient: (Δ(mcrC-mrr)114::IS10) Wild type E. coli B strains carry a Type I restriction endonuclease which cleaves DNA with the site TGA(N8)TGCT. While E. coli DNA is protected from degradation by a cognate methyl-transferase, foreign DNA will be cut at these sites. The deletion described above eliminates both the methylase and the endonuclease. Methyl Restriction Deficient: (Δ(mcrC-mrr)114::IS10 and R(mcr-73::miniTn10--TetS)2): E. coli has a system of enzymes encoded by mcrA, mcrBC and mrr which will cleave DNA with methylation patterns found in higher eukaryotes, as well as some plant and bacterial strains. All three Mcr enzymes and Mrr have been inactivated in T7 Express allowing the introduction of eukaryotic DNA of genomic origin (e.g. primary libraries) if desired. T1 Phage Resistant: (fhuA2) T1, an extremely virulent phage requires the E. coli ferric hydroxamate uptake receptor for infectivity. Deletion of this gene confers resistance to this type of phage, but does not significantly affect the transformation or growth characteristics of the cell.