  The nucleotide composition of human genes with a special emphasis on
transcription-related strand asymmetries is analyzed. Such asymmetries may be
associated with different mutational rates in two principal factors. The first
one is transcription-coupled repair and the second one is the selective
pressure related to optimization of the translation efficiency. The former
factor affects both coding and noncoding regions of a gene, while the latter
factor is applicable only to the coding regions. Compositional asymmetries
calculated at the third position of a codon in coding (exons) and noncoding
(introns, UTR, upstream and downstream) regions of human genes are compared. It
is shown that the keto-skew (excess of the frequencies of G and T nucleotides
over the frequencies of A and C nucleotides in the same strand) is most
pronounced in intronic regions, less pronounced in coding regions, and has near
zero values in untranscribed regions. The keto-skew correlates with the level
of gene expression in germ-line cells in both introns and exons. We propose to
use the results of our analysis to estimate the contribution of different
evolutionary factors to the transcription-related compositional biases.
