The structure of the DNA binding domain, determined at 1.8 angstrom resolution, contains a three-helix bundle that is capped by a four-stranded antiparallel beta sheet. This structure is a variant of the helix-turn-helix motif, typified by catabolite activator protein. In the heat shock transcription factor, the first helix of the motif (alpha 2) has an alpha-helical bulge and a proline-induced kink. The angle between the two helices of the motif (alpha 2 and alpha 3) is about 20 degrees smaller than the average for canonical helix-turn-helix proteins. Nevertheless, the relative positions of the first and third helices of the bundle (alpha 1 and alpha 3) are conserved. It is proposed here that the first helix of the three-helix bundle be considered a component of the helix-turn-helix motif.
Genetically identical populations of unicellular organisms often show marked variation in some phenotypic traits. To investigate the molecular causes and possible biological functions of this phenotypic noise, it would be useful to have a method to identify genes whose expression varies stochastically on a certain time scale. Here, we developed such a method and used it for identifying genes with high levels of phenotypic noise in Salmonella enterica ssp. I serovar Typhimurium (S. Typhimurium). We created a genomic plasmid library fused to a green fluorescent protein (GFP) reporter and subjected replicate populations harboring this library to fluctuating selection for GFP expression using fluorescent-activated cell sorting (FACS). After seven rounds of fluctuating selection, the populations were strongly enriched for promoters that showed a high amount of noise in gene expression. Our results indicate that the activity of some promoters of S. Typhimurium varies on such a short time scale that these promoters can absorb rapid fluctuations in the direction of selection, as imposed during our experiment. The genomic fragments that conferred the highest levels of phenotypic variation were promoters controlling the synthesis of flagella, which are associated with virulence and host–pathogen interactions. This confirms earlier reports that phenotypic noise may play a role in pathogenesis and indicates that these promoters have among the highest levels of noise in the S. Typhimurium genome. This approach can be applied to many other bacterial and eukaryotic systems as a simple method for identifying genes with noisy expression.
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