Dynamic interplay between antagonistic pathways controlling the sigma super(32) level in Escherichia coli

• Published in: Proceedings of the National Academy of Sciences - PNAS 2000-05, Vol.97 (11), p.5860-5865
• Main Authors: Morita, M T, Kanemori, M, Yanagi, H, Yura, T
• Format: Article
• Language: English
• Subjects: Escherichia coli; rpoH gene; sigma factor s32
• ISSN: 0027-8424
• DOI: 10.1073/pnas.080495197

The heat-shock response in Escherichia coli depends primarily on the transient increase in the cellular level of heat-shock sigma factor sigma super(32) encoded by the rpoH gene, which results from both enhanced synthesis and transient stabilization of normally unstable sigma super(32). Heat-induced synthesis of sigma super(32) was previously shown to occur at the translation level by melting the mRNA secondary structure formed within the 5' coding sequence of rpoH including the translation initiation region. The subsequent decrease in the sigma super(32) level during the adaptation phase has been thought to involve both shutoff of synthesis (translation) and destabilization of sigma super(32)-mediated by the DnaK-DnaJ chaperones, although direct evidence for translational repression was lacking. We now show that the heat-induced synthesis of sigma super(32) does not shut off at the translation level by using a reporter system involving translational coupling. Furthermore, the apparent shutoff was not observed when the synthesis rate was determined by a very short pulse labeling (15 s). Examination of sigma super(32) stability at 10 min after shift from 30 to 42 degree C revealed more extreme instability (t sub(1/2)=20 s) than had previously been thought. Thus, the dynamic change in sigma super(32) stability during the heat-shock response largely accounts for the apparent shutoff of sigma super(32) synthesis observed with a longer pulse. These results suggest a mechanism for maintaining the intricate balance between the antagonistic pathways: the rpoH translation as determined primarily by ambient temperature and the turnover of sigma super(32) as modulated by the chaperone (and presumably protease)-mediated autogenous control.