Is Carl Woese losing a Kingdom?

Graham Clark G.CLARK at lshtm.ac.uk
Mon Sep 30 04:32:12 EST 1996

lamoran at gpu.utcc.utoronto.ca (L.A. Moran) writes:

>One of the main reasons that gene X has been used for phylogeny 
>reconstruction is the idea that it is vertically transmitted and
>not subject to lateral gene transfer.
>You can plug anything you want into "gene X". All such statements
>have about the same scientific legitimacy. Why do some people feel 
>that rRNA genes are special in this sense? 

There ARE good reasons, mostly revolving around co-evolution. The 
rRNAs do not exist in the cell on their own but as part of an 
organelle. While most proteins intract with one or two others
in their funtional role in the cell, the ribosome is made up of
dozens of distinct proteins many of which are involved in specific
protein-rRNA interactions that involve specific structures/sequences
in both molecules. In other words expressing a heterologous rRNA in
a cell is unlikely to give rise to functional ribosomes, unless it
comes from a closely related organism. In addition to the multiple
coevolved rRNA/protein interactions forming a barrier to horizontal
transfer, it has been shown that rRNA promoter and RNA polymerase I 
also coevolve rapidly in eukaryotes, such that heterologous expression
is not possible because of promoter incompatibility even between quite
closely related organisms. 

So while coevolution will affect the successful horizontal transfer of
many genes, the multiplicity of the inter-molecular interactions in
which ribosomal RNAs are involved means that functional products from 
such an event are highly unlikely.


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