Abstract:
BACKGROUND : The order Enterobacterales encompasses a broad range of metabolically and ecologically versatile
bacterial taxa, most of which are motile by means of peritrichous flagella. Flagellar biosynthesis has been linked to a
primary flagella locus, flag-1, encompassing ~ 50 genes. A discrete locus, flag-2, encoding a distinct flagellar system,
has been observed in a limited number of enterobacterial taxa, but its function remains largely uncharacterized.
RESULTS : Comparative genomic analyses showed that orthologous flag-2 loci are present in 592/4028 taxa
belonging to 5/8 and 31/76 families and genera, respectively, in the order Enterobacterales. Furthermore, the
presence of only the outermost flag-2 genes in many taxa suggests that this locus was far more prevalent and has
subsequently been lost through gene deletion events. The flag-2 loci range in size from ~ 3.4 to 81.1 kilobases and
code for between five and 102 distinct proteins. The discrepancy in size and protein number can be attributed to
the presence of cargo gene islands within the loci. Evolutionary analyses revealed a complex evolutionary history
for the flag-2 loci, representing ancestral elements in some taxa, while showing evidence of recent horizontal
acquisition in other enterobacteria.
CONCLUSIONS : The flag-2 flagellar system is a fairly common, but highly variable feature among members of the
Enterobacterales. Given the energetic burden of flagellar biosynthesis and functioning, the prevalence of a second
flagellar system suggests it plays important biological roles in the enterobacteria and we postulate on its potential
role as locomotory organ or as secretion system.
Description:
Additional file 1: Table S1. Presence/absence of flag-2 loci among
4028 strains belonging to eight families and 72 genera. The presence of
flag-1 loci is also indicated. The previous taxonomy denotes the
taxonomy according to the NCBI genome database, while the current
taxonomy is according to the Genome Taxonomy Database (GTDB). The
isolation source as well as habitat/lifestyles of the different strains are
given. Table S2. Molecular characteristics of the flag-2 loci among 592
taxa in the Enterobacterales. The sizes of the flag-2 loci, variable regions
VR1 and VR2, their G + C contents (%) and G + C deviation (%) from the
genome are shown. The number of predicted proteins encoded on each
of these flag-2 fractions are also shown. Table S3. Characteristics of the
cargo genes encoded in the variable regions VR1 and VR2 and elsewhere
in the enterobacterial flag-2 loci. The number of strains and the families/
genera in which each protein occurs within the flag-2 loci are indicated,
as well as the average amino acid identities among enterobacterial
orthologues. Conserved domains present in each cargo protein as
determined by BlastP analysis against the Conserved Domain Database
are shown.
