Xylella fastidiosa is the etiologic agent of a wide range of plant diseases, including citrus variegated chlorosis (CVC), a major threat to citrus industry. The genomes of several strains of this phytopathogen were completely sequenced, enabling large-scale functional studies. DNA microarrays representing 2,608 (91.6%) coding sequences (CDS) of X. fastidiosa CVC strain 9a5c were used to investigate transcript levels during growth with different iron availabilities. When treated with the iron chelator 2,2-dipyridyl, 193 CDS were considered up-regulated and 216 were considered down-regulated. Upon incubation with 100 M ferric pyrophosphate, 218 and 256 CDS were considered up-and down-regulated, respectively. Differential expression for a subset of 44 CDS was further evaluated by reverse transcription-quantitative PCR. Several CDS involved with regulatory functions, pathogenicity, and cell structure were modulated under both conditions assayed, suggesting that major changes in cell architecture and metabolism occur when X. fastidiosa cells are exposed to extreme variations in iron concentration. Interestingly, the modulated CDS include those related to colicin V-like bacteriocin synthesis and secretion and to functions of pili/fimbriae. We also investigated the contribution of the ferric uptake regulator Fur to the iron stimulon of X. fastidiosa. The promoter regions of the strain 9a5c genome were screened for putative Fur boxes, and candidates were analyzed by electrophoretic mobility shift assays. Taken together, our data support the hypothesis that Fur is not solely responsible for the modulation of the iron stimulon of X. fastidiosa, and they present novel evidence for iron regulation of pathogenicity determinants.When challenged with limiting iron concentrations such as those encountered in host tissues, pathogenic bacteria commonly manage iron homeostasis by releasing iron from intracellular reservoirs and increasing the expression of iron acquisition systems (73). This allows survival in an otherwise prohibiting environment, since iron is an essential cofactor for many proteins mediating electron transfer and redox reactions. Besides functioning as a barrier against pathogens, light control of free iron concentration within host tissues reduces the deleterious effects of iron overload, such as the generation of reactive oxygen species (78,83). Strict control of iron metabolism is also observed for bacteria, and in most cases it is exerted by the ferric uptake regulator, Fur. When bound to Fe 2ϩ , this intracellular iron sensor is capable of binding to Fur boxes at operator sequences, blocking the transcription of genes involved in many cellular processes besides iron uptake (1). The description of the iron regulatory circuitry has recently increased in complexity due to the discovery of other transcriptional repressors and of small noncoding regulatory RNAs (23,51,82).Many pathogenic bacteria have also evolved to couple the iron-limiting response to the expression of other virulence determinants, such as exo...
