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J. Bacteriol., Oct 1997, 6391-6399, Vol 179, No. 20
J Platzer, W Sterr, M Hausmann and R Schmitt
The peritrichous flagella of Rhizobium meliloti rotate only clockwise and
control directional changes of swimming cells by modulating flagellar
rotary speed. Using Tn5 insertions, we have identified and sequenced a
motility (mot) operon containing three genes, motB, motC, and motD, that
are translationally coupled. The motB gene (and an unlinked motA) has been
assigned by similarity to the Escherichia coli and Bacillus subtilis
homologs, whereas motC and motD are new and without known precedents in
other bacteria. In-frame deletions introduced in motB, motC, or motD each
result in paralysis. MotD function was fully restored by complementation
with the wild-type motD gene. By contrast, deletions in motB or motC
required the native combination of motB and motC in trans for restoring
normal flagellar rotation, whereas complementation with motB or motC alone
led to uncoordinated (jiggly) swimming. Similarly, a motB-motC gene fusion
and a Tn5 insertion intervening between motB and motC resulted in jiggly
swimming as a consequence of large fluctuations in flagellar rotary speed.
We conclude that MotC biosynthesis requires coordinate expression of motB
and motC and balanced amounts of the two gene products. The MotC
polypeptide contains an N-terminal signal sequence for export, and Western
blots have confirmed its location in the periplasm of the R. meliloti cell.
A working model suggests that interactions between MotB and MotC at the
periplasmic surface of the motor control the energy flux or the energy
coupling that drives flagellar rotation.
Copyright © 1997, American Society for Microbiology
Three genes of a motility operon and their role in flagellar rotary speed variation in Rhizobium meliloti
Lehrstuhl fur Genetik, Universitat Regensburg, Germany.
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