Heat shock transcription factor σ 32 defective in membrane transport can be suppressed by transposon insertion into genes encoding a restriction enzyme subunit or a putative autotransporter in Escherichia coli.

Heat shock transcription factor σ32 of Escherichia coli plays a major role in protein homeostasis and requires membrane localization for regulation. We here report that a strongly deregulated I54N-σ32 mutant defective in association with the membrane can be phenotypically suppressed by Tn5 insertion into the mcrC or ydbA2 gene, encoding a restriction enzyme subunit or part of a putative autotransporter, respectively. The suppression is specific for mutant I54N-σ32 and reduces its activity but not its abundance or stability. Moreover, the deregulated phenotype of I54N-σ32 is effectively suppressed by a plasmid carrying the same mcrC::Tn5 mutation. In contrast, deletion of the mcrC or ydbA2 gene hardly affects I54N-σ32 activity. These results, taken together, suggest that the truncated form of McrC (and presumably also of YdbA2) protein produced by the Tn5 insertion interacts specifically with I54N-σ32 to reduce its activity without substantially affecting its amount or stability.