## 1 Introduction  
_Leptospira interrogans_ is one of the many pathogenic species of the genus _Leptospira_ and is composed of several strains that can cause leptospirosis in mammals, including humans. It has been reported that over 1 million human cases of severe leptospirosis occur worldwide each year, with approximately 60,000 deaths from this disease [1]. It has to be noted that leptospirosis also generates huge economic losses in a number of countries due to reproductive disorders in cattle, sheep, pigs, and horses [2; 3]. Furthermore, recent serological and microbiological studies have indicated a high rate of leptospiral infections in domestic animals [2; 4; 5; 6]. Despite a high risk of leptospirosis, especially in the tropical and subtropical countries, and its global importance, molecular mechanisms of both the leptospiral virulence and the disease pathogenesis currently remain largely unknown [7; 8], mainly due to the historical lack of standard genetic tools for use in work with the pathogenic _Leptospira_ species. Recent advances in genetic manipulation of these species have made it possible to identify several leptospiral virulence factors. However, many of them have turned out not to be required for virulence in animal models [7]. Unfortunately, the limitations of modern genetic tools available for pathogenic _Leptospira_ spp. still have an enormous impact on the understanding of the molecular and cellular mechanisms involved in the pathogenesis of leptospirosis. Like other pathogenic bacteria, _L. interrogans_ is exposed to environmental stresses during infection of mammalian hosts. Bacteria fight various environmental stressors by altering the expression of genes involved in host adaptation and promoting their survival. Transcriptional regulation, and especially sigma factors controlling the promoter selectivity of bacterial RNA polymerase, play a crucial role in this stress-induced gene expression response. Unfortunately, there is limited information on gene regulation in _Leptospira_ spp. Comparative genomics and genome-wide in silico analyses had shown that the _Leptospira_ genome contains one basic sigma factor-encoding gene- _rpoD_ (\(\sigma^{70}\)), and alternative sigma factor genes: _rpoF_ (\(\sigma^{28}\)), _rpoN_ (\(\sigma^{54}\)), and several (5-11) genes encoding extracytoplasmic function (ECF) sigma factors, referred to as _rpoE_ (\(\sigma^{\text{E}}\)) in the literature [9; 10] (Table 1). According to these analyses, the housekeeping sigma factor, \(\sigma^{70}\), coordinates transcription of most genes in the leptospiral cells, while alternative sigma factors play key roles in environmental adaptation of bacteria and their virulence. In that light, it is intriguing that the _Leptospira_ species differ in the number of ECF \(\sigma\) factors. For example, pathogenic _L. interrogans_ encodes ten different ECF \(\sigma\) factors (Table 2), while saprophytic species encode only five [9]. On the other hand, the well-known bacterium _E. coli_ possesses only two ECF \(\sigma\) factors [14] (Table 1). It is postulated that the ECF \(\sigma\) factors existing only in the pathogenic _Leptospira_ species are important for the _Leptospira_ life cycle within the host [9]. Unraveling regulation of virulence genes' expression is a particularly important challenge because it is necessary for understanding the molecular basis of the disease caused by the pathogen and its underpinning. Molecular chaperone ClpB, a member of the Hsp100/Clp subfamily of the AAA + ATPases (_ATPases associated with a variety of cellular activities_), is among the known leptospiral virulence factors. It has been demonstrated that ClpB deficiency in _L. interrogans_ resulted in bacterial growth defects under oxidative and heat stresses, and also in its loss of virulence [15]. Furthermore, in previous studies, we had shown that _L. interrogans_ ClpB (ClpB\({}_{\text{Li}}\)) is not only synthesized but is also immunogenic during the infection process, further supporting its involvement in _Leptospira_ pathogenicity [16].