Escherichia coli tktA and tktB genes encode two transketolase isoenzymes involved in the pentose-phosphate pathway, In this study, two reporter lacZ fusions, tktA- and tktB-lacZ, were constructed to examine their transcriptional regulation on the E. coli chromosome. The tktA gene was induced in the exponential growth phase and suppressed in the stationary growth phase. However, the genetic elimination of the rpoS, whose product is an alternative sigma factor (RpoS), derepressed the tktA gene expression in the stationary growth phase, indicating that the RpoS sigma factor negatively regulates the tktA gene expression in the stationary growth phase. On the contrary, the tktB gene expression showed the highest value in the stationary growth phase and the RpoS positively regulated the tktB gene expression in the stationary growth phase. We also verified the role of the RpoS affecting the regulation of the tktA and tktB gene expression by the reverse transcription (RT)-PCR experiments. These results suggest that the differential growth- dependent expressions of the tktA and tktB genes are caused by the RpoS action.