F. Togashi et al., AN EXTREME CLOCKWISE SWITCH BIAS MUTATION IN FLIG OF SALMONELLA-TYPHIMURIUM AND ITS SUPPRESSION BY SLOW-MOTILE MUTATIONS IN MOTA AND MOTB, Journal of bacteriology, 179(9), 1997, pp. 2994-3003
Pseudorevertants (second-site suppressor mutants) were isolated from a
set of parental mutants of Salmonella with defects in the flagellar s
witch genes fliG and fliM. Most of the suppressing mutations lay in fl
agellar region IIIb of the chromosome, One fliG mutant, SJW2511, gave
rise to a large number of suppressor mutations in the motility genes m
otA and motB, which are in flagellar region IL, SJW2811, which has a t
hree-amino-acid deletion (Delta Pro-Ala-Ala) at positions 169 to 171 o
f FliG, had an extreme clockwise motor bias that produced inverse smoo
th swimming (i,e,, swimming by means of clockwise rotation of a hydrod
ynamically induced right-handed helical bundle), and formed Mot(-)-lik
e colonies on semisolid medium. Unlike previously reported inverse-swi
mming mutants, it did not shaw a chemotactic response to serine, and i
t remained inverse even in a Delta che background; thus, its switch is
locked in the clockwise state, The location of the mutation further u
nderscores the conclusion from a previous study of spontaneous missens
e mutants (V.M. Irikura, M, Kihara, S, Yamaguchi, H, Sockett, and R, M
, Macnab, J, Bacteriol, 175:802-810, 1993) that a relatively localized
region in the central part of the FliG sequence is critically importa
nt for switching, All of the second-site mutations in motA and motB ca
used some impairment of motility, both in the pseudorevertants and in
a wild-type fliG background, The mechanism of suppression of the fliG
mutation by the mot mutations is complex, involving destabilization of
the right-handed flagellar bundle as a result of reduced motor speed,
The mutations in the MotA and MotB sequences were clustered to a cons
iderable degree as follows: in transmembrane helices 3 and 4 of MotA a
nd the sole transmembrane helix of MotB, at helix-membrane interfaces,
in the cytoplasmic domains of MotA, and in the vicinity of the peptid
oglycan binding region of the periplasmic domain of MotB. The potentia
l importance of Lys28 and Asp33 of the MotB sequence for proton delive
ry to the site of torque generation is discussed.