Resumo

Iron is an essential micronutrient, participating on the structure of many important proteins. Bacteria secrete iron chelators (siderophores) and regulate iron uptake genes according to its availability in the environment. Part of this gene regulation is made by iron starvation sigma factors, which drive transcription of genes for siderophore transport or biosynthesis. They are usually regulated by Fur, a transcriptional repressor which utilizes Fe²⁺ as a co-repressor. Contrary to other bacteria, there are no Fur-binding sites upstream of the genes encoding putative iron starvation sigma factors from Caulobacter crescentus, and it is not known if they are involved in iron uptake. We observed by β-galactosidase activity assays that there was no significant induction of the genes upon iron starvation, neither in a fur mutant strain, indicating that they are not directly regulated by iron availability or by Fur. Mutant strains for two of the putative anti-sigma factors genes (here named fecR2 and fecR4) were constructed, as a way of turning on the corresponding sigma factor activity. Their growth curves were similar to those of the parental strain on different iron sources, and equally affected by the iron chelator 2,2-dipyridyl, indicating that the mutations did not affect growth in both excess and depletion of iron sources. DNA microarray experiments revealed that the only gene upregulated in mutant ∆fecR2 in comparison to the parental strain encodes its corresponding TonB-dependent receptor, suggesting that this is the only target gene for the FecI2 sigma factor. The ∆fecR4 strain showed upregulation of at least 70 genes, including some involved in aminoacid biosynthesis and utilization of alternative carbon sources. However, phenotype microarrays using 95 carbon sources showed no significant growth differences between the parental and the ∆fecI4 strain. The results suggest that the two sigma factors control different sets of target genes, being the FecI2 sigma factor-mediated response very specific, whereas FecI4 seems to regulate a more general response.