Previous studies have shown that several bacterial species exhibit a multigenic response following temperature downshift (cold shock). Evidence for such a response in is reported, based on the isolation of a range of low-induction-temperature gene fusions containing Mu insertions. The fusions exhibited different levels of basal light at 30 °C, and were induced at different rates and to different degrees over several hours following a reduction in temperature to 10 °C. Of the Mu gene fusions isolated, one was found which produced essentially no light when grown at 30 °C but exhibited rapid and high-level induction when the temperature was reduced to 10 °C. The target of this gene fusion (which was named ) was shown to lie adjacent to the operon and to encode a homologue of the major cold-shock protein of , CspA. Luminescence studies revealed that substantial light production occurred from the ::Mu fusion at or below 22 °C but not at higher temperatures, even following a temperature drop from 30 °C. Moreover, mRNA levels were found to mimic this pattern of luminescence, suggesting that expression occurs below a defined temperature threshold. The mRNA was also found to be very stable at 10 °C but to become highly unstable when the temperature was raised towards the threshold temperature, even in the presence of rifampicin. Existing cellular RNases therefore appear to mediate the decay of mRNA at high temperatures, but are incapable of this at low temperatures.


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