Several bacterial genomes exhibit preference for G over C on the DNA leading 
strand extending from the origin of replication to the ter-region in the genomes 
of Escherichia coli, Mycoplasma genitalium, Bacillus subtilis, and marginally in 
Haemophilus influenzae, Mycoplasma pneumoniae, and Helicobacter pylori. Strand 
compositional asymmetry is not observed in the cyanobacterium Synechocystis sp. 
genome nor in the archaeal genomes of Methanococcus jannaschii, Methanobacterium 
thermoautotrophicum, and Archaeoglobus fulgidus. A strong strand compositional 
asymmetry is observed in beta-type but not alpha- or gamma-type human 
herpesviruses featuring G > C downstream of oriL and C > G upstream of oriL. 
Dinucleotide relative abundances (i.e., dinucleotide representations normalized 
by the component nucleotide frequencies) are consonant with respect to the 
leading and lagging strands. Strand compositional asymmetry may reflect on 
differences in replication synthesis of the leading versus lagging strand, on 
differences between template and coding strand associated with 
transcription-coupled repair mechanisms, on differences in gene density between 
the two strands, on differences in residue and codon biases in relation to gene 
function, expression level, or operon organization, or on differences in single 
or context-dependent base mutational rates. The absence of strand asymmetry in 
the archaeal genomes may reflect the presence of multiple origins of 
replication.