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).