4.2) Rsm/Csr regulation by H-NS homologs and IHF
Different NAPs can have similar functions in different species. An
example of this is the Csr/Rsm sRNA family, which regulates the
CsrA/RsmA proteins and is regulated by either the H-NS family of
proteins or IHF, depending on the species (Figure 2 ). Indeed,
the central component of the carbon storage regulator (Csr) and the
homologous repressor of secondary metabolites (Rsm) systems is an RNA
binding protein (CsrA or RsmA) that inhibits protein translation by
binding to a stem-loop RNA motif in the 5’-region of its mRNA targets,
such as genes involved in metabolism, motility, biofilm formation and
quorum sensing and/or pathogenesis. sRNAs such as CsrB/RsmB, CsrC, RsmZ,
RsmY and RsmX (the names vary depending on the bacterial species),
contain multiple sequence/structural motifs that mimic the RNA motif and
sequester the CsrA/RsmA protein. These sRNAs have a redundant titration
function, but distinct regulatory pathways. In P. aeruginosa PAO1, the GacS/GacA system transduces regulatory signals to
downstream genes by directly controlling the expression of only two
genes, rsmY and rsmZ . Castang et al., (2008) showed that
MvaT and MvaU, two members of the H-NS family, interact with rsmZ but not with rsmY . The repression of RsmZ transcription by MvaT
was validated using mutants defective for the mvaT andmvaU genes, and gene fusions (Brencic et al. , 2009).
However, in P. fluorescens, in vitro experiments showed that the
AT-rich promoter-linker region of the rsmZ gene has two IHF
binding sites, suggesting that IHF is likely to be involved in the
regulation of rsmZ rather than the H-NS homologues (Humair,
Wackwitz and Haas, 2010). In Salmonella , the sRNAs that
sequester CsrA are CsrB and CsrC, which are similar to those in P.
fluorescens, as is the regulatory mechanism whereby IHF activates the
transcription of csrB but it has no effect on the transcriptional
regulation of csrC (Martínez et al. , 2014). Furthermore,
in E. amylovora, IHF positively regulates the rsmB sRNA to
control motility (Lee and Zhao, 2016). CsrA homologs and Csr/Rsm sRNAs
are present in most Gammaproteobacteria species, includingVibrio species and Legionella pneumophila , as well as thePseudomonas species, Salmonella enterica ,Xanthomonas spp. and Erwinia carotovora , suggesting that
the regulation illustrated here involving either H-NS homologs or IHF
may be widespread in Gammaproteobacteria species, including
pathogenic and beneficial biocontrol species.