## 1 Introduction  
In order to survive and colonize specific niches within their hosts, bacterial pathogens must rapidly respond to environmental signals and adjust the expression pattern of cellular factors involved in its pathogenesis. For this purpose, regulatory systems that control and coordinate the expression of functionally related subsets of genes are required. Chemically diverse molecules, such as proteins, regulatory RNAs, and low molecular weight non-proteinaceous molecules, also known as alarmones, can act as regulators of those regulatory circuits. The level of the alarmones can be quickly altered by specific enzymatic activities, thus mediating rapid changes in the pattern of gene expression. Guanosine tetrahedra and pentaphophate, hereafter referred to as ppGpp, are modified nucleotides that are synthesized by the bacterial cells in response to different environmental signals as a result of unfavorable growth conditions [1; 2]. In _Escherichia coli_, ppGpp turnover depends on two enzymes: the ribosome-associated ReLA synthetase and the SpoT protein, which has both synthetase and hydrolase activities [1]. In _E. coli_, under standard laboratory conditions, ppGpp levels are low during exponential growth (40 uM), increasing about 20-fold upon entry into stationary phase and later stabilizing at 150 M [3]. ppGpp, initially described as a mediator of the global metabolic response to amino acid starvation (the stringent response), is also involved in the regulation of many other cellular functions related with the adaptation of bacterial cells to environmental stresses [1; 4; 5; 6]. Hence, it has been described that ppGpp controls the expression of genes involved in the ability of bacterial pathogens to colonize their hosts [7; 8; 9].  
Uropathogenic _E. coli_ (UPEC) causes urinary tract infections (UTIs). UPEC expresses several adhesins enabling bacterial cells to colonize different parts of the urinary tract and, often, UPEC also produces toxins such as the \(\alpha\)-hemolysin and the cytotoxic necrotizing factor [10].  
The secreted \(\alpha\)-hemolysin plays a key role during UTI by causing cytolytic and cytotoxic effects in a wide range of mammalian cell types [11; 12; 13]. The \(\alpha\)-hemolysin belongs to the family of RTX toxins characterized by some conserved structural features and by sharing a common organization of their genetic determinants. The _hlyCABD_ operon codes for genes that enable bacterial cells to synthesize, activate, and secrete the \(\alpha\)-hemolysin [14]. The precursor of the active toxin (HlyA, molecular mass 110 kDa), is activated in the cytoplasm by HlyC, a fatty acid acyltransferase [15]. The active \(\alpha\)-hemolysin is directly secreted from the cytoplasm to the extracellular milieu through a transmembrane channel consisting of three proteins, namely, HlyB, HlyD, and ToIC [16]. The UPEC isolate J96, isolated from a pyelonephritis patient, encodes two _hlyCABD_ operons, denoted I and II, located within two independent pathogenicity islands [17; 18]. The _hlyCABD\({}_{II}\)_ operon has been extensively studied and was used in fundamental studies demonstrating the existence of virulence factors in bacteria [11].  
In previous reports, it was demonstrated that ppGpp plays a crucial role in the control of the expression of important colonization factors during the establishment of UTI by UPEC, such as type 1 fimbiae, antigen 43, and flagella [19; 20; 21]. Here, we found that this alarmone controls also the expression of the _hlyCABD\({}_{II}\)_ operon. The amount of \(\alpha\)-hemolysin secreted and its associated cytotoxicity is importantly diminished in a strain deficient in ppGpp production. Our data support the assumption that ppGpp coordinates the expression of cellular factors required during an UTI process.