In eukaryotes, frataxin deficiency (FXN) causes severe phenotypes including loss 
of iron-sulfur (Fe-S) cluster protein activity, accumulation of mitochondrial 
iron and leads to the neurodegenerative disease Friedreich's ataxia. In 
contrast, in prokaryotes, deficiency in the FXN homolog, CyaY, was reported not 
to cause any significant phenotype, questioning both its importance and its 
actual contribution to Fe-S cluster biogenesis. Because FXN is conserved between 
eukaryotes and prokaryotes, this surprising discrepancy prompted us to 
reinvestigate the role of CyaY in Escherichia coli. We report that CyaY (i) 
potentiates E. coli fitness, (ii) belongs to the ISC pathway catalyzing the 
maturation of Fe-S cluster-containing proteins and (iii) requires iron-rich 
conditions for its contribution to be significant. A genetic interaction was 
discovered between cyaY and iscX, the last gene of the isc operon. Deletion of 
both genes showed an additive effect on Fe-S cluster protein maturation, which 
led, among others, to increased resistance to aminoglycosides and increased 
sensitivity to lambda phage infection. Together, these in vivo results establish 
the importance of CyaY as a member of the ISC-mediated Fe-S cluster biogenesis 
pathway in E. coli, like it does in eukaryotes, and validate IscX as a new bona 
fide Fe-S cluster biogenesis factor.