Beschreibung:
<jats:p>
To study adaptive evolution in defined environments, we performed evolution experiments with
<jats:italic>Saccharomyces cerevisiae</jats:italic>
(yeast) in nitrogen-limited chemostat cultures. We used DNA microarrays to identify copy-number variation associated with adaptation and observed frequent amplifications and deletions at the
<jats:italic>GAP1</jats:italic>
locus.
<jats:italic>GAP1</jats:italic>
encodes the general amino acid permease, which transports amino acids across the plasma membrane. We identified a self-propagating extrachromosomal circular DNA molecule that results from intrachromosomal recombination between long terminal repeats (LTRs) flanking
<jats:italic>GAP1</jats:italic>
. Extrachromosomal DNA circles (
<jats:italic>GAP1</jats:italic>
<jats:sup>circle</jats:sup>
) contain
<jats:italic>GAP1</jats:italic>
, the replication origin
<jats:italic>ARS1116</jats:italic>
, and a single hybrid LTR derived from recombination between the two flanking LTRs. Formation of the
<jats:italic>GAP1</jats:italic>
<jats:sup>circle</jats:sup>
is associated with deletion of chromosomal
<jats:italic>GAP1</jats:italic>
(
<jats:italic>gap1Δ</jats:italic>
) and production of a single hybrid LTR at the
<jats:italic>GAP1</jats:italic>
chromosomal locus. The
<jats:italic>GAP1</jats:italic>
<jats:sup>circle</jats:sup>
is selected following prolonged culturing in
<jats:sc>l</jats:sc>
-glutamine–limited chemostats in a manner analogous to the selection of oncogenes present on double minutes in human cancers. Clones carrying only the
<jats:italic>gap1Δ</jats:italic>
allele were selected under various non-amino acid nitrogen limitations including ammonium, urea, and allantoin limitation. Previous studies have shown that the rate of intrachromosomal recombination between tandem repeats is stimulated by transcription of the intervening sequence. The high level of
<jats:italic>GAP1</jats:italic>
expression in nitrogen-limited chemostats suggests that the frequency of
<jats:italic>GAP1</jats:italic>
<jats:sup>circle</jats:sup>
and
<jats:italic>gap1Δ</jats:italic>
generation may be increased under nitrogen-limiting conditions. We propose that this genomic architecture facilitates evolvability of
<jats:italic>S. cerevisiae</jats:italic>
populations exposed to variation in levels and sources of environmental nitrogen.
</jats:p>