A Unique cis-Encoded Small Noncoding RNA Is Regulating Legionella pneumophila Hfq Expression in a Life Cycle-Dependent Manner

作者：Oliva Giulia; Sahr Tobias; Rolando Monica; Knoth Maike; Buchrieser Carmen

来源：mBio, 2017, 8(1): e02182-16.

DOI：10.1128/mBio.02182-16

<jats:title>ABSTRACT</jats:title>
<jats:p>
<jats:italic>Legionella pneumophila</jats:italic>
is an environmental bacterium that parasitizes protozoa, but it may also infect humans, thereby causing a severe pneumonia called Legionnaires’ disease. To cycle between the environment and a eukaryotic host,
<jats:italic>L. pneumophila</jats:italic>
is regulating the expression of virulence factors in a life cycle-dependent manner: replicating bacteria do not express virulence factors, whereas transmissive bacteria are highly motile and infective. Here we show that Hfq is an important regulator in this network. Hfq is highly expressed in transmissive bacteria but is expressed at very low levels in replicating bacteria. A
<jats:italic>L. pneumophila hfq</jats:italic>
deletion mutant exhibits reduced abilities to infect and multiply in
<jats:italic>Acanthamoeba castellanii</jats:italic>
at environmental temperatures. The life cycle-dependent regulation of Hfq expression depends on a unique
<jats:italic>cis</jats:italic>
-encoded small RNA named Anti-hfq that is transcribed antisense of the
<jats:italic>hfq</jats:italic>
transcript and overlaps its 5′ untranslated region. The Anti-hfq sRNA is highly expressed only in replicating
<jats:italic>L. pneumophila</jats:italic>
where it regulates
<jats:italic>hfq</jats:italic>
expression through binding to the complementary regions of the
<jats:italic>hfq</jats:italic>
transcripts. This results in reduced Hfq protein levels in exponentially growing cells. Both the small noncoding RNA (sRNA) and
<jats:italic>hfq</jats:italic>
mRNA are bound and stabilized by the Hfq protein, likely leading to the cleavage of the RNA duplex by the endoribonuclease RNase III. In contrast, after the switch to transmissive bacteria, the sRNA is not expressed, allowing now an efficient expression of the
<jats:italic>hfq</jats:italic>
gene and consequently Hfq. Our results place Hfq and its newly identified sRNA anti-
<jats:italic>hfq</jats:italic>
in the center of the regulatory network governing
<jats:italic>L. pneumophila</jats:italic>
differentiation from nonvirulent to virulent bacteria.
</jats:p>
<jats:p>
<jats:bold>IMPORTANCE</jats:bold>
The abilities of
<jats:italic>L. pneumophila</jats:italic>
to replicate intracellularly and to cause disease depend on its capacity to adapt to different extra- and intracellular environmental conditions. Therefore, a timely and fine-tuned expression of virulence factors and adaptation traits is crucial. Yet, the regulatory circuits governing the life cycle of
<jats:italic>L. pneumophila</jats:italic>
from replicating to virulent bacteria are only partly uncovered. Here we show that the life cycle-dependent regulation of the RNA chaperone Hfq relies on a small regulatory RNA encoded antisense to the
<jats:italic>hfq</jats:italic>
-encoding gene through a base pairing mechanism. Furthermore, Hfq regulates its own expression in an autoregulatory loop. The discovery of this RNA regulatory mechanism in
<jats:italic>L. pneumophila</jats:italic>
is an important step forward in the understanding of how the switch from inoffensive, replicating to highly virulent, transmissive
<jats:italic>L. pneumophila</jats:italic>
is regulated.
</jats:p>

• 出版日期2017-2