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. 2012 May;194(10):2586-93.
doi: 10.1128/JB.05567-11. Epub 2012 Mar 16.

The FsrA sRNA and FbpB protein mediate the iron-dependent induction of the Bacillus subtilis lutABC iron-sulfur-containing oxidases

Affiliations

The FsrA sRNA and FbpB protein mediate the iron-dependent induction of the Bacillus subtilis lutABC iron-sulfur-containing oxidases

Gregory T Smaldone et al. J Bacteriol. 2012 May.

Abstract

The Bacillus subtilis ferric uptake regulator (Fur) protein regulates iron homeostasis and directly represses more than 20 operons. Fur indirectly regulates many more genes, including those controlled by the small, noncoding RNA FsrA. FsrA translationally represses numerous target genes and, for at least some targets, appears to function in conjunction with one or more of three small, basic proteins, known as FbpA, FbpB, and FbpC. The lactate-inducible lutABC operon encodes iron sulfur-containing enzymes required for growth on lactate. We here demonstrate that a fur mutant strain grows poorly on lactate due to FsrA-dependent repression of LutABC synthesis. Growth is restored in an fsrA mutant and also partially restored by mutation of the fbpAB operon. Genetic studies indicate that the 48-amino-acid FbpB protein but not FbpA contributes to regulation of lutABC. FbpB may function, at least in part, by increasing the efficiency of FsrA targeting to the lutABC mRNA, since the role of FbpB can be bypassed by modest upregulation of FsrA. These results provide support for a model in which FbpB, and perhaps other Fbp proteins, contributes along with FsrA to the translational regulation of gene expression.

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Figures

Fig 1
Fig 1
Growth on lactate minimal medium is reduced in a fur mutant and is restored by mutation in either fsrA or fbpAB. Growth curves of the indicated strains in lactate MM are shown. (A) WT (strain CU1065, open diamond), fur (strain HB2501, cross), fur fsrA (strain HB5733, open triangle), fur fbpAB (strain HB5735, open circle), fur fsrA fbpAB (strain HB5751, open square), fur fsrA+fsrA (strain HB12510, filled triangle), and fur fbpAB+fbpAB (strain, HB12511, filled circle) strains were used. (B) fur (strain HB2501, cross), fur fsrA (strain HB5733, open triangle), fur fbpAB (strain HB5751, open circle), and fur fbpAB+fsrA (strain HB12594, filled square) strains were used.
Fig 2
Fig 2
Repression of LutABC expression involves both FsrA and FbpAB. Western blot analysis of LutA-FLAG (top panel), LutB-FLAG (middle), and LutC-FLAG (bottom) expression in mutant strains was carried out with 10 μg total crude extract loaded per lane (an unidentified cross-reactive band that also serves as a loading control is indicated). Each lane is labeled with the relevant genotype of the strain background. A wild-type strain carrying no FLAG tag construct is in the left lane as a negative control. Signal intensities were quantified using ImageQuant software.
Fig 3
Fig 3
Regulation of LutA-FLAG expression requires translation of FbpB but not FbpA. (A) Illustration of the start codon mutations generated in an ectopically integrated copy of the fbpAB operon. (B) Western blotting of LutA-FLAG in various mutant strains was carried out with 10 μg total crude extract loaded per lane. Load Cont., loading control. Strains used in lanes 1 to 8 are CU1065, HB12524, HB12532, HB12533, HB12579, HB12580, HB12581, and HB12582, respectively.
Fig 4
Fig 4
Induction of the FbpB-FLAG protein is sufficient to restore repression of LutA-FLAG expression in an fbpAB mutant. Western blotting of LutA-FLAG was carried out with 10 μg total crude extract loaded per lane. Strains used were HB12524, HB12532, HB12533, and HB7498 (A) and HB12524, HB12532, HB12533, and HB7497 (B). Each lane is labeled with the mutant background, and xyl. ind. indicates the presence of xylose-inducible FbpB or FbpA. Where indicated, 2% xylose was added (xyl. ind.) to induce expression of FbpB-FLAG (A) or FbpA-FLAG (B).
Fig 5
Fig 5
Northern blot analysis of lutA and FsrA. Equal amounts (2 μg) of total RNA isolated from various strains were hybridized with antisense RNA probes specific for the lutA-FLAG mRNA and FsrA as indicated. Cells were grown in Belitsky minimal medium and harvested during logarithmic growth (OD500 of 0.5). Numbered columns correspond to the strains as indicated, where “ACG A” indicates a start codon mutation for FbpA and “ACG B” indicates a start codon mutation for FbpB (as for Fig. 3). Sizes of the specific transcripts are given. Note that in lane 1, the full-length lutABC transcript was too large to be resolved and would not be apparent. Strains used in lanes 1 to 10 are HB2501, HB12534, HB12526, HB12529, HB12532, HB12533, HB12579, HB12580, HB12581, and HB12582, respectively.
Fig 6
Fig 6
Colony morphology on lactate medium. Colony morphology of NCIB 3610-derived strains as observed on MSgg medium (top pane) or MSgg with 0.5% lactate in place of glycerol. Images shown were taken after 120 h of growth at room temperature.
Fig 7
Fig 7
Regulation of lutABC operon expression by components of the iron-sparing response. (A) Illustration of the predicted pairing between the lutABC 5′UTR (above) and the sRNA FsrA (below). The RBS is indicated by the solid black bar, and the AUG start codon is boxed. RNAs are numbered relative to the transcription start site (+1). (B) Summary of the multiple levels of regulation at the lutABC operon. Arrows represent the processes of transcription and translation, and bars represent repression at either the transcriptional or posttranscriptional level. FbpB may work to facilitate FsrA annealing or by other pathways, not yet defined (indicated by “?”).

References

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