In <1995Jul9.124845.39635 at ac.dal.ca> arlin at ac.dal.ca writes:
>Some eukaryotic genes are composed of historically separate genes that
>"live together" in the form of gene fusions. For instance, Neurospora
>and many other fungi have a "trp1" gene that is homologous to three
>seaparate genes (trpG, trpF and trpC) of Pseudomonas putida and many
>other bacteria (see Hutter, et al. 1986, Annu. Rev. Microbiol. 40, 55).
> Prokaryotes also have trp gene fusions (see Crawford, 1989 Annu. Rev.
>Microbiol 43, 567). For instance, E. coli has a "trpD" gene that is
>homologous to trpG and trpD of P. putida. My impression is that, in
>general, eukaryotes are more likely than prokaryotes to have fused
>genes. The reasons for this difference between eukaryotes and
>prokaryotes, and the reasons for the fusions, are not well understood.
Since it isn't already decided, I'd like to speculate.
First, could it be possible that some organism ancestral to fungi and
pseudomonas might have had the fused genes, and what we're seeing is
gene fission? If so, one possible reason is that the separate genes
can be regulated independently. If one of them has a poor Vmax or Km
then they can make more of that one. This sort of balancing would be
most important to small organisms that need to wring extreme efficiency
from their enzymes.
Possible values for fused genes include: Simpler regulation -- you
don't need two sets of gene regulators, one will do. Automatic location
-- reactions proceed fastest when the next enzyme in line is very close,
since diffusion time is minimised. Fused enzymes are close. Automatic
transport -- they need only one mechanism to transport them to the
right location in the cell (eucaryotes only).
Only the first of these reasons can work for fused genes that produce
separate products. If the mRNA is processed to to produce only one
protein fragment, then arguments that require a single large protein
One more -- Long ago, some alternate alleles for these genes might have
arisen that worked well together but not apart. (They may have arisen
in one subpopulation, and then spread into another.) Selection would
favor them if they were linked, but not otherwise. If the first
closely-linked version to arise was the fused version, that could
spread widely. But this doesn't explain the fused E coli genes, unless
we say it happened in a sexual ancestor of both the bacteria and the
fungi, and the P putida fission happened later. It seems farfetched
for this example, but the explanation might be workable for some other