In article <009883B6.756109C0.83 at hal.hahnemann.edu>
mcgeed at HAL.HAHNEMANN.EDU writes:
> Path: news.uh.edu!swrinde!pipex!uunet!olivea!biosci!HAL.HAHNEMANN.EDU!mcgeed
> From: mcgeed at HAL.HAHNEMANN.EDU> Newsgroups: bionet.microbiology
> Subject: Suppression of Rho termination=suppression of nonsense mutant by tRNA?
> Date: 30 Nov 1994 05:38:03 -0800
> Organization: BIOSCI International Newsgroups for Molecular Biology
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>> You wrote:
> > In article <a-281194182438 at 128.174.101.140>, a () wrote:
> >
> > > I am somewhat confused. Is suppression of rho termination the same as
> > > suppression of a nonsense mutant by tRNA?
> > >
> > > king1 at uxa.cso.uiuc.edu> > >
> > > Thanks
> > hello a ()
> > from what I understand, your so call suppression of nonsense mutant by
> > tRNA actually occur because mutation in specific tRNA that lead to it
> > recognizing wrong codon which in this case is the stop codon, thus allow
> > continue transcription pass the nonsense stop codon. example of such
> > mutants in bact. are amber, and at least 2 more which I forgot. Any how,
> > suppresion of rho termination is due to a different mechanism, and it is a
> > mode of transcriptional regulation rather than due to a mutation.
> >
> Fellow microbionetters:
> A nonsense mutation in tRNA-amino acid occurs in the anticodon,
> resulting in loss of base-pairing between the anticodon and the codon
> and leading to a possible halt in translation (not transcription). I
> say "possible" because there are other tRNAs for the same amino acid
> (for example, there might be 4 or so tRNAs for serine, with each one
> having a different anticodon sequence). Thus a nonsense mutation in
> the tRNA possibly results in no amino acid being put in the growing
> polypeptide chain during translation. A suppression of this mutation
> can occur by a compensatory change in the codon sequence, such that the
> base-pairing between the codon and anticodon is restored (thus
> restoring any translation that has halted). I think this is different
> than nonsense mutations in the mRNA (codon), where a previously
> functional codon is mutated to a nonsense codon (UAA, UAG, or UGA),
> leading to a definite halt in translation of the message. Suppression
> of this mutation is by a compensatory change in the anticodon in the
> tRNA that recognized the original codon sequence. The three nonsense
> mutations are called amber, ochre and opal, although I can't remember
> which one goes to which stop codon.
> Rho-dependent termination is a phenomenon of transcription (not
> translation). This is suppressed by proteins called antiterminators.
> These proteins would then allow transcription to keep going. As
> mentioned above by someone else, this suppression doesn't necessarily
> have to be due to mutation, since anti-terminators exist (Go Mr. Arnold
> Schwarzenegger!), although I suppose that mutations in rho itself, for
> example, could suppress rho-dependent termination.
>> Comments and certainly corrections are welcome.
>> David
>>>> David J. McGee
>MCGEED at hal.hahnemann.edu>> Graduate Student
> McPHU (Medical College of Pennsylvania and Hahnemann University)
> Center City Campus
> 15th and Vine Streets
> Dept. Microbiology and Immunology M.S. 410
> New College Building RM 10302
> Philadelphia, PA 19102
>> Phone: 215-762-8275
> Fax: 215-762-1004
>
The above answer is correct in detail, but not quite complete. It is
possible for tRNA suppression and rho suppression to have similar
phenotype. That is when you consider the case of an operon. IF you have
a nonsense mutation early in the first gene, for example, of an
operon, it will not only abolish expression of that gene but also may
significantly reduce, if not abolish, expression of the downstream
non-mutant genes due to polarity. Transcription of these genes is
reduced. a tRNA suppressor which restores expression of the first
mutant gene will also restore expression of the downstream genes.
However a rho mutant will not correct the nonsense mutation defect, but
it will restore expression of the downstream genes. It will supress the
polarity defect of the nonsense mutation.
So the answer depends upon whether you discuss suppression of a
specific mutation or suppression of polarity in an operon.
Michael Benedik benedik at uh.edu
Biochemical Sciences
University of Houston
Houston, TX 77204-5934
