Suppression of Rho termination=suppression of nonsense mutant by tRNA?
mcgeed at HAL.HAHNEMANN.EDU
mcgeed at HAL.HAHNEMANN.EDU
Wed Nov 30 08:38:03 EST 1994
> In article <a-281194182438 at 220.127.116.11>, 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.
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 J. McGee
MCGEED at hal.hahnemann.edu
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
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