In addition to its role in rRNA maturation, the importance of bacterial RNase III was highlighted through transcriptomic studies performed in diverse organisms, including but not limited to
E. coli [49][50][51][52],
Streptomyces coelicolor [53],
Staphylococcus aureus [54],
Synechococcus sp. PCC7002
[55] or
Rhodobacter sphaeroides [56]. These studies demonstrate the pleiotropic role of RNase III in the control of gene expression and a comparison of the genes affected by RNase III inactivation in these organisms would be informative about the distribution and conservation (or not) of targets. In
E. coli, RNase III is involved in the destabilization of numerous RNAs. For example, Ec-RNase III cleaves its own mRNA in the 5′UTR of the
rnc-
era mRNA, which destabilizes the whole transcript (
Figure 3)
[37]. Ec-RNase III can cleave in between coding sequences within polycistronic mRNAs such as
rpsO-
pnp, encoding the ribosomal protein S15 and the exoribonuclease PNPase. This cleavage leads to the destabilization of
pnp mRNA without affecting the expression of
rpsO (
Figure 3). Cleavages within coding sequences were also found as in the
arfA mRNA (
Figure 3), encoding the alternative ribosome rescue factor ArfA, thus revealing a positive role of Ec-RNase III in an alternative pathway to rescue stalled ribosomes upon mRNA truncation
[57]. While targets of RNase III in bacteria are usually expected to be negatively regulated, maturation can also lead to positive regulation as in the case of the pre-rRNA (see
Section 4.1). RNase III is also involved in intermolecular dsRNA cleavages (i.e., where the dsRNA is composed of two distinct molecules) as in the case of regulatory RNAs bound to their targets. For the small RNA RhyB, binding to the
sodB mRNA, encoding the superoxide dismutase FeSOD, RNase III cleavage leads to the degradation of both RNAs
[58] while, on the other hand, the cleavage of the antisense RNA (asRNA) ArrS bound to the
gadE mRNA leads to increased translation of GadE, an acid response transcriptional factor
[59]. These and other examples demonstrate the pleiotropic functions of RNase III in bacterial physiology. For example, in the adaptation phase following an osmotic shock RNase III activity is repressed, which allows stabilization of
proP (
Figure 3),
proU and
betT mRNAs encoding proteins involved in the import of osmoprotectants
[60][61][62]. Furthermore, RNase III was shown to be involved in thermotolerance
[19], motility
[63] and aminoglycoside resistance
[64]. In other bacteria, RNase III was shown to be important for a whole range of functions, including but not limited to methionine biosynthesis in
S. aureus [65] and cell wall homeostasis in
Pseudomonas putida [66] and to be involved in virulence in
Enterococcus faecalis [67],
Listeria monocytogenes [68],
S. aureus [54] or
Campylobacter jejuni [69].
Although the majority of characterized RNase III target sites in
E. coli likely result from intramolecular dsRNA, it was recently shown that among the thousands of in vivo Ec-RNase III cleavage sites identified, around 40% are singletons, in the sense that there is no obvious staggered second cleavage site. Hence, this suggests that they either represent single-stranded cleavages or arise from intermolecular interactions (i.e., the second single-strand cleavage is located in a second molecule thus a complex analysis is required to predict candidate dsRNA partners)
[49]. The plasticity of RNase III binding and cleavage sites in
E. coli, as illustrated in
Figure 3 for the
rnc-era,
rpsO-pnp,
arfA and
proP mRNAs, may provide an explanation for the abundance of putative RNase III cleavage sites (identified by transcriptomic approaches but not yet validated as direct targets) and is consistent with a larger role of RNase III in the regulation of gene expression.