A significant number of these modulators are involved in the adaptation to nutritional stimuli (Guillouard et al., 2002). Others are involved in the response of bacterial cells to environmental outputs, such as stress (Lahiri et al., 2008) or quorum sensing (Cao et al., 2001; Kim et al., 2004). Some of them are of special interest due to their implication in virulence (Axler-Diperte et al., 2006; Heroven & Dersch, 2006). A recent

report has shown that Salmonella encodes 44 LTTRs. To date, the target genes of just 16 of them have been characterized (Lahiri et al., 2009). We present here preliminary characterization of GSK-3 beta phosphorylation YfeR (referred as STM2424 by Lahiri et al., 2009). Evidence that this modulator belonging to the LTTR family comes from the facts that YfeR (1) exhibits sequence and structure similarities to members of the LTTR, (2) binds specifically to the intergenic yfeR/yfeH region and (3) autoregulates its own transcription. An outstanding feature of YfeR is the fact that

its expression is sensitive to the osmolarity of the medium, and it is induced when cells grow at low osmolarity. MAPK Inhibitor Library Apart this report, low osmotic stress increasing the expression of a LTTR has only been described for the Anabaena regulator RbcR1 (Mori et al., 2002). A global transcriptomic analysis of Yersinia pestis showed three osmolarity-regulated LTTRs (upregulated by high-salinity stress; Han et al., 2005). Interestingly, expression of the putative Na+-dependent transporter YfeH appears to be governed by a complex network, rather than by YfeR alone. It is apparent that, rather than osmolarity, the main factor influencing YfeH expression is the stationary phase. Expression of yfeH increases in yfeR mutants only when cultures grown at high osmolarity enter the stationary mafosfamide phase. Whereas this suggests that YfeR is a repressor of yfeH transcription, it is also apparent that factors other than YfeR modulate

YfeH expression. In turn, this strongly suggests that, as has been shown for other LTTRs, YfeR targets genes other than to those adjacent to it, and that may be required for optimal growth under low osmolarity conditions. The global transcriptomic analysis performed in this work supports this assumption. Interestingly, whereas several upregulated genes in the yfeR mutant encode envelope proteins, downregulated genes encode proteins related to amino acid transport and metabolism. These results suggest that, when growing under low osmolarity conditions, YfeR, either directly or indirectly, represses some envelope proteins and induces specific amino acid metabolic pathways. These factors may contribute to the adaptive response of Salmonella to low osmolarity conditions. Whereas in the recent past LTTRs appeared to specifically modulate transcription of their adjacent gene, increasing experimental evidence shows that both modulators and the modulated genes are related to more complex regulatory networks (Lehnen et al.