5.4) Lrp is the target of several regulations by sRNA
The expression level of E. coli lrp is controlled by different proteins and environmental conditions. H-NS and the nitrite-sensitive repressor NsrR could repress the transcription of thelrp gene (Oshima et al. , 1995; Partridge et al. , 2009), whereas GadE and the alarmone ppGpp activate the expression oflrp during amino acid starvation (Hommais et al. , 2004; Traxler et al. , 2011). Furthermore, the lrp gene is autoregulated (Wang et al. , 1994), and it has been reported that arginine-loaded ArgR indirectly interferes with the negative autoregulation of lrp (Torres Montaguth et al. , 2019).
In E. coli and S. enterica, the location of the transcription start site is more than 250 nt upstream of the start codon, making this 5’UTR a desirable site for sRNA binding and regulation (Wang et al. , 1994; McFarland and Dorman, 2008). Wright et al., (2013) developed CopraRNA, an algorithm that uses comparative genomics to predict mRNA targets for bacterial small RNAs. Indeed, they identified lrp as a putative target of 7 sRNAs: GcvB, MicF, DsrA, FnrS, MicA, MicC, and RprA-L. Experimental validation was performed on GcvB, MicF and DsrA. In addition, a novel sRNA called ArcZ, was discovered to post-transcriptionally repress lrp inErwinia amylovora (Figure 4a-b ) (Schachterle and Sundin, 2019). These regulations will be reviewed below.