A versatile and general prokaryotic expression vector, pLACT7.

We have previously reported the constitutive over-expression of the tRNA-guanine transglycosylase (TGT) from plasmid pTGT1 in Escherichia coli. To obtain a controllable expression system for TGT, we have subsequently cloned the tgt gene into pET21b. Though the overexpression of TGT is inducible in pET21b, the plasmid has a low copy number, a poor yield of single-stranded DNA and relies on an E. coli strain that produces T7 RNA polymerase for protein expression. We have combined the features of pTZ18U and pET21b and have constructed a versatile plasmid pLACT7 that has a high copy number, a high yield of single-stranded DNA and both the T7 and lac promoters for protein expression in a wide variety of E. coli strains.