>From: Chris=Michiels%LMM%AGR at agr.kuleuven.ac.be
>Hello,
>Does anybody have, or know references of, E. coli membrane
>transition temperature mutants? What is (are) their phenotype(s)?
>What are the genes involved and their functions? Does membrane
>transition temperature vary a lot with growth conditions? A lot of
>questions, but any quantum of answer is appreciated!
>Chris Michiels
>chris.michiels at agr.kuleuven.ac.be
Chris,
The membrane fluidity of E. coli is determined in large part by the
ratio of unsaturated to saturated fatty acids (UFA:SFA) in the
phospholipids. At 37C, this ratio is about 0.5 (if I remember
correctly) and goes up to 16 at 10C. The basal level of UFA
synthesised is determined by the enzymes B-ketoacyl-ACP synthase I
(encoded by _fabB_) and B-hydroxydecanoyl-ACP dehydrase (_fabA_).
Strains with mutations in either of these loci require
supplementation with UFA. The FabA protein introduces the double
bond into the growing acyl-chain when it is ten carbons long. The
FabB protein catalyses the condensation of growing saturated or
unsaturated acyl chains with malonyl-ACP to form B-ketoacyl-ACP 2
carbons longer.
B-ketoacyl-ACP synthase II, encoded by _fabF_, catalyses the key step
in thermal regulation of the fatty acid composition. This enzyme
elongates palmitoleoyl-ACP [16:1-ACP (number carbons:number double
bonds)] to cis-vaccenoyl-ACP (18:1). (16:1-ACP is a very poor
substrate for the FabB enzyme). Strains with a defective fabF grow
normally, but do not contain 18:1 and are defective in temperature
regulation of the menbrane composition. The FabF enzyme is also
*more* active at lower temperatures, leading to an increase in the
amount of 18:1 in the phospholipids, and hence more fluid
membranes.
The fatty acid compostion of membranes thus changes with
temperature in wild type strains, and so, yes, the transition
temperature will vary according to growth temperature. _fabF_
strains will have the same TT at all temps. There is also a strain
which overexpresses _fabF_ (termed _Vtr_, I believe) which has
increased levels of 18:1 in the membranes, and hence altered TT.
Overexpression of _fabB_ or _fabA_ does not cause much of a change
in the UFA:SFA ratio or levels of 18:1... I think. You'll have to
check on that ;-)
A couple of refs to get you started...
K. Magnuson, S. Jackowski, C.O. Rock, and J.E. Cronan Jr (1993)
"Regulation of fatty acid biosynthesis in E. coli" Microbiological
Reviews, 57(3), pp522-542.
[A good general ref on FA biosynthesis - has a bit on thermal
regulation too. Check out the Garwin refs in this for stuff
on fabF. Oh, and I work for Chuck Rock...]
F.C Neidhardt (1987) "E. coli and S. Typhimurium. Cellular and
molecular biology" ASM Press, Washington DC
[A book every microbiologist should have....]
A.G Marr and J.I Ingraham (1962) J. Bacteriol., 84, 1260-1267.
[An old one. This field goes back a long way!]
I hope that this helps somewhat. Drop me a line if you need some
more info.
Richard Heath, D.Phil
Department of Biochemistry
St Jude Children's Research Hospital,
Memphis, TN, 38101
heath at mbcf.stjude.org
PS: I thought people didn't worry about transition temperatures
any more...
pps: I have had a bugger of a time trying to do a followup posting of this
(which I wrote on the wp, then ftp'd). Hence, this posting. Not a followup,
but it should make it :-)
