In an Escherichia coli coupled transcription-translation system, expression of the osmoregulated gene proU is stimulated at elevated potassium concentrations and by an extract from cells grown at high osmolality.

We have studied the in vitro expression of the osmoregulated proU promoter in Escherichia coli coupled transcription-translation (S-30) extracts as a function of the osmolality of the culture medium used to grow the cells and of the salt concentration added to the extract. These variables represent novel extensions of the use of S-30 extracts to investigate gene regulatory phenomena in vitro. The concentrations of potassium acetate and of the physiologically relevant osmolyte potassium glutamate for maximal expression of the proU promoter are approximately 2-fold higher than the concentrations of these salts providing maximal expression of the lacUV5 promoter. The relative promoter activity of proU with respect to lacUV5 increases more than 30-fold with an increase in salt concentration from 50 to 300 mM. In comparative studies with S-30 extracts prepared from cells grown at high and low osmolalities, we find that at fixed salt concentrations expression of proU is increased 10-fold in the S-30 extract prepared from high osmolality-grown cells, whereas the expression of lacUV5 is increased less than 2-fold. Addition of anti-sigma 70 monoclonal antibodies or purified sigma 70 to the S-30 extract demonstrated that the major proU promoter(s) used in the S-30 extracts is sigma 70-dependent.