5.4.1) Lrp and GcvB
GcvB is a 200 nt long Hfq-associated sRNA that is highly conserved in Gram-negative bacteria (Zhang et al. , 2003; Sharma et al. , 2007). It regulates amino acid metabolism and transport and short peptide transport by repressing several ABC transporters, including theoppABCDF and dppABCDF operons (Pulvermacher, Stauffer and Stauffer, 2008, 2009; Sharma et al. , 2011). In addition, GcvB directly interacts with the 5’UTR region of lrp mRNA (Figure 4c ) (Lee and Gottesman, 2016). The sRNA consists of four stem-loops (SL) defining three regions: R1, R2 and R3 (Sharmaet al. , 2011). R1 is a 30-nt-long G/U-rich single-stranded sequence that separates SL1 and SL2 and regulates 92% of the GcvB target mRNA (Lalaouna et al. , 2019). The R2 is a single-stranded decamer between SL3 and SL4. Although R1 and R2 are highly conserved, they are not required for lrp binding. However, the R3 region contains a CUGUC sequence that is crucial for the lrp binding (Lee and Gottesman, 2016). Indeed, the R3 portion of GcvB protects two GACAG regions on the lrp mRNA leader, located between −179 and −175 nucleotides and between −39 and −33 nucleotides from the ATG, respectively (Figure 4c ) (Lee and Gottesman, 2016). The presence of two GcvB binding sites on the lrp mRNA suggests that two GcvB molecules are required for the maximal repression of lrp in rich medium conditions and under oxidative stress (Figure 4a ) (Lee and Gottesman, 2016). However, it is still unknown how GcvB/lrp pairing leads to regulation. Although for the binding site located between -39 and -33 nt, it has been suggested that GcvB interferes with a transcriptional enhancer located between -69 and -40 nt (Figure 4c ), the regulation for the other site remains unexplained. Nevertheless, the structure of the lrp leader may be critical for efficient translation, as certain mutations can interfere with translation and/or GcvB regulation (Lee and Gottesman, 2016).