Regional base composition variation along yeast chromosome III:
evolution of chromosome primary structure
Paul M. Sharp and Andrew T. Lloyd
Nucleic Acids Research ,
21(2), 179-183 (1993)
Abstract
The recent determination of the complete sequence of chromosome III
from the yeast Saccharomyces cerevisiae allows, for the first time, the investigation of
the long range primary structure of a eukaryotic chromosome. We have found that,
against a background G+C level of about 35%, there are two regions (one in each
chromosome arm) in which G+C values rise to over 50%. This effect is seen in silent
sites within genes, but not in noncoding intergenic sequences. The variation in G+C
content is not related to differential selection of synonymous codons, and probably
reflects mutational biases. That the intergenic regions do not exhibit the same
phenomenon is particularly interesting, and suggests that they are under substantial
constraint. The yeast chromosome may be a model of the structure of the human
genome, since there is evidence that it is also a mosaic of long regions of different
base compositions, reflected in wide variation of G+C content at silent sites among
genes. Two possible causes of this regional effect, replication timing, and
recombination frequency, are discussed.
