Temperature-dependent retention of a tryptophan-operon-bearing plasmid in Escherichia coli.

Derivatives of plasmid pBR322 carrying the tryptophan operon are rapidly lost from a growing culture. The valS gene from Escherichia coli was combined with the tryptophan operon on a pBR322 derivative, pSGS21. In a thermosensitive valS mutant of E. coli K-12 host, plasmid pSGS21 was stably inherited for over 200 generations at temperatures nonpermissive for the chromosomal mutation. At 30 degrees C, a temperature at which the host valSts protein is functional, the plasmid was lost at a rate of 1.2% per cell generation.