Abstract
To measure the length of heating required to convert a λ lysogen in the immune (im+) phase into the anti-immune (im-) phase, rex gene activity was used as an indicator. It was observed that 5 min heating at 41 °C did not shift any lysogenic cells of 594(λN-cI857O-) from the im+ phase into the im- phase, and it took 17 min heating at 41 °C followed by long hours of culture at 30 °C to shift half of the lysogenic cells into the im- phase. Such a length of heating is too long to be accounted for by blocking of the expression of lambda repressor by cro gene product. The result is more consistent with accumulation of a certain level of cro gene product during heating so that the synthesis of the repressor is blocked even after a return to low temperature.