Molecular Biology and Evolution, Vol 5, 377-391, Copyright © 1988 by Society for Molecular Biology and Evolution
JM Hancock and GA Dover
The set of "expansion segments" of any eukaryotic 26S/28S ribosomal RNA
(rRNA) gene is responsible for the bulk of the difference in length between
the prokaryotic 23S rRNA gene and the eukaryotic 26S/28S rRNA gene. The
expansion segments are also responsible for interspecific fluctuations in
length during eukaryotic evolution. They show a consistent bias in base
composition in any species; for example, they are AT rich in Drosophila
melanogaster and GC rich in vertebrate species. Dot-matrix comparisons of
sets of expansion segments reveal high similarities between members of a
set within any 28S rRNA gene of a species, in contrast to the little or
spurious similarity that exists between sets of expansion segments from
distantly related species. Similarities among members of a set of expansion
segments within any 28S rRNA gene cannot be accounted for by their
base-compositional bias alone. In contrast, no significant similarity
exists within a set of "core" segments (regions between expansion segments)
of any 28S rRNA gene, although core segments are conserved between species.
The set of expansion segments of a 26S/28S gene is coevolving as a unit in
each species, at the same time as the family of 28S rRNA genes, as a whole,
is undergoing continual homogenization, making all sets of expansion
segments from all ribosomal DNA (rDNA) arrays in a species similar in
sequence. Analysis of DNA simplicity of 26S/28S rRNA genes shows a direct
correlation between significantly high relative simplicity factors (RSFs)
and sequence similarity among a set of expansion segments. A similar
correlation exists between RSF values, overall rDNA lengths, and the
lengths of individual expansion segments. Such correlations suggest that
most length fluctuations reflect the gain and loss of simple sequence
motifs by slippage-like mechanisms. We discuss the molecular coevolution of
expansion segments, which takes place against a background of slippage-like
and unequal crossing-over mechanisms of turnover that are responsible for
the accumulation of interspecific differences in rDNA sequences.
ORIGINAL ARTICLE
Molecular coevolution among cryptically simple expansion segments of eukaryotic 26S/28S rRNAs
Genetics Department, University of Cambridge, United Kingdom.
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