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Analysis of the N-terminal leucine heptad and hexad repeats of sigma 54.

In order to assess the role of leucine repeat motifs within bacterial protein sigma 54, a series of point mutants were introduced into the many leucine residues near the N terminus. Functional assays in vivo showed that the leucine residues that comprise the previously identified heptad repeat motif are selectively important for function. These heptad leucine residues are critical for mRNA production and also for recognition of the -12 promoter element. An internal proline substitution destroys the function of the heptad repeat region, suggesting a possible alpha-helical structure. Mutants with changes in the distal part of this N-terminal region show the interesting property of allowing nearly full levels of open complex formation, while nonetheless reducing the level of mRNA transcripts produced. All of the above-mentioned properties differ from those exhibited by mutating the interdigitated glutamine residues, which were previously found to be closely involved in the DNA melting reaction. The collection of data suggests that the N-terminal region contains overlapping functional motifs, hydrophobic heptad and glutamine-rich, which together appear to constitute the activation domain of sigma 54.