Evidence that GroEL, not sigma 32, is involved in transcriptional regulation of the Vibrio fischeri luminescence genes in Escherichia coli

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Abstract
In Escherichia coli, transcription of the inducible Vibrio fischeri luminescence operon, luxICDABE, has been reported to require sigma 32, the product of rpoH. Consistent with previous studies, we report that an E. coli delta rpoH mutant, KY1601 containing luxICDABE and luxR, which codes for the activator of luxICDABE transcription on a plasmid (pJE202), was weakly luminescent. Transformation of this E. coli strain with a plasmid containing rpoH under the control of the tac promoter resulted in high levels of cellular luminescence. However, the level of expression of the pJE202 luxICDABE was also high in E. coli 1603, a delta rpoH mutant with a second-site mutation that resulted in sigma 32-independent overexpression of the groE operon. Apparently, sigma 32 is not directly required for the transcription of luxICDABE in E. coli but is required for sufficient expression of groE, which is in turn required for the transcription of luxICDABE. This conclusion is supported by the finding that E. coli groE mutants containing pJE202 were weakly luminescent. In the E. coli delta rpoH mutant KY1601, the sigma 32 requirement for the transcription of luxICDABE was partially compensated for by the addition of saturating concentrations of the inducer to the culture medium and largely compensated for when cells were transformed with a luxR overexpression vector. These data support the hypothesis that sigma 32 is not required for transcription of luxICDABE. Rather, it appears that the products of groE are required for the folding of LuxR into an active protein, like they are for the folding of several other proteins.