To discover new factors that are involved in iron acquisition by
Legionella pneumophila
, we used RNA-Seq to identify the genes that are most highly induced when virulent strain 130b is cultured in a low-iron chemically defined medium. Among other things, this revealed
14915
, a heretofore uncharacterized gene that is predicted to be transcriptionally regulated by Fur and to encode a novel, ~15 kDa protein.
14915
was present in all
L. pneumophila
strains examined and had homologs in a subset of the other
Legionella
species. Compatible with it containing a classic signal sequence, the 14915 protein was detected in bacterial culture supernatants in a manner dependent upon the
L. pneumophila
type II secretion system. Thus, we designated 14915 as IrsA for
ir
on-
r
egulated,
s
ecreted protein
A
. Based on mutant analysis, the
irsA
gene was not required for optimal growth of strain 130b in low-iron media. However, after discovering that the commonly used laboratory-derived strain Lp02 has a much greater requirement for iron, we uncovered a growth-enhancing role for IrsA after examining an Lp02 mutant that lacked both IrsA and the Fe
2+
-transporter FeoB. The
irsA
mutant of 130b, but not its complemented derivative, did, however, display increased biofilm formation on both plastic and agar surfaces, and compatible with this, the mutant hyper-aggregated. Thus, IrsA is a novel, iron-regulated exoprotein that modulates biofilm formation and, under some circumstances, promotes growth in low-iron conditions. For this study, we determined and deposited in the database a complete and fully assembled genome sequence for strain 130b.
IMPORTANCE
The bacterium
Legionella pneumophila
is the principal cause of Legionnaires’ disease, a potentially fatal form of pneumonia that is increasing in incidence.
L. pneumophila
exists in many natural and human-made water systems and can be transmitted to humans through inhalation of contaminated water droplets.
L. pneumophila
flourishes within its habitats by spreading planktonically, assembling into biofilms, and growing in larger host cells. Iron acquisition is a key determinant for
L. pneumophila
persistence in water and during infection. We previously demonstrated that
L. pneumophila
assimilates iron both by secreting a non-protein iron chelator (siderophore) and by importing iron through membrane transporters. In this study, we uncovered a novel, secreted protein that is highly iron-regulated, promotes
L. pneumophila
’s growth in low-iron media, and impacts biofilm formation. We also identified uncharacterized, IrsA-related proteins in other important human and animal pathogens. Thus, our results have important implications for understanding iron assimilation, biofilm formation, and pathogenesis.
