How do bacteria sense env.?

Daniel Mytelka mytelka at zenith.Berkeley.EDU
Sun Sep 4 19:39:01 EST 1994


>On 2 Sep 1994, Karen Ferri wrote:
>
>> 
>> Hello all.  Am taking my first micro. class and am absolutely fascinated 
>> by it!  I am having a little trouble understanding the mech. by which 
>> bacteria sense their environment.  I understand the mech. of chemotaxis; 
>> it's how do they sense in the first place I am confused on and haven't 
>> been able to find any clear info if at all ( my text omits this).  Some 
>> of the terms  the prof used were: omp R, omp C, omp F, sensor in the 
>> cytoplasmic membrane and response regulator in the cytoplasm.  Can anyone 
>> explain this to me in clear language?  Many thanks, in advance.
>> 
>> KF
>> 
>> 
>
>

We seem to be talking about E. coli here.  E. coli are surrounded by two
membranes, an outer membrane and an inner membrane. The outer membrane
is fairly porous, containing general "porins" which will allow pretty
much anything below a certain size through. The two main porins are ompF
and ompC, with the one expressed at any particular time being governed
by a number of factors which feed into the transcriptional regulator
ompR. ompR is your "response regulator in the cytoplasm" (ie it is
inside the cell rather than stuck in or between the membranes). It is one of
a family of "two component systems", in which one component moniters
something and signals the other at an appropriate time to create some
response. In this case, the protein envZ is the "sensor in the
cytoplasmic membrane" (ie it is stuck in the inner membrane), monitering
the external osmolarity and directing ompR accordingly (ie it checks
whether there is a lot of molecules outside the cell which will flow in
if there are a lot of large pores). As far as regulating motility and
chemotaxis, the outer membrane may as well not be present, since the
pores allow all relavent molecules through.

The important thing about sensing for chemotaxis is realizing that it is
a temporal phenomenom. In other words, the bacteria compare the
concentration of interesting molecules at times x and x+y, not at one
specific time. They have a number of receptors which can bind to these
interesting compounds, allowing the bacteria to get an overall value
of their current surroundings: if the receptors are all bound to
the molecule they recognize, it must be abundant; if less are occupied,
it is less abundant. They then modify their receptors in a way
which allows them to "remember" this value. A moment later, they check
again and compare: if there are more good things around, they are going
in a good direction; if there are fewer, they aren't. The major 
receptors are also "sensors in the cytoplasmic membrane" and also 
interact with "response regulators in the cytoplasm" which help
direct the cell.

 
Daniel Mytelka
Department of Molecular and Cell Biology, Berkeley
mytelka at mendel.berkeley.edu
 



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