Question: Between which types of DNA bases are mutational biases introduced due to directional mutation pressure?

Context:
directional mutation pressure for variation of the DNA G + C content due to mutational biases between alpha-bases (A or T) and gamma-bases (G or C)
A/T-biased directional mutation pressure
Rates of substitution mutations in two directions, v [from an A-T or T-A nucleotide pair (AT-pair) to a G-C or C-G nucleotide pair (GC-pair)] and u [from a GC-pair to an AT-pair], are usually not the same. The net effect, v/(u + v), has previously been defined as directional mutation pressure (mu D), which explains the wide interspecific variation and narrow intragenomic heterogeneity of DNA G + C content in bacteria.
The prokaryotic genetic code has been influenced by directional mutation pressure (GC/AT pressure) that has been exerted on the entire genome.
directional mutation pressure affecting the base composition of DNA, sometimes in the direction of increased GC content and at other times, in the direction of AT.
GC content of DNA varies, as a result of directional mutation pressure (AT/GC pressure), especially in bacteria.
the effect of mutation on a genome is not random but has a directionality toward higher or lower guanine-plus-cytosine content of DNA, and this pressure generates directional changes more in neutral parts of the genome than in functionally significant parts.
GC-biased mutation pressure
Directional mutation theory predicts that when the mutational bias between A/T and G/C nucleotide pairs is equilibrated with the base composition of a neutral set of DNA nucleotides, the mutation frequency per gene will be much lower than the frequency immediately after the mutator mutation takes place.
The parity rules lay the foundation for evaluating the biases in synonymous codon usage in terms of (1) directional mutation pressure for variation of the DNA G + C content due to mutational biases between alpha-bases (A or T) and gamma-bases (G or C), (2) strand-bias mutation, for example, by DNA repair during transcription, and (3) functional selection in evolution, for example, due to tRNA abundance.
Directional mutation theory predicts that when the mutational bias between A/T and G/C nucleotide pairs is equilibrated with the base composition of a neutral set of DNA nucleotides, the mutation frequency per gene will be much lower than the frequency immediately after the mutator mutation takes place.
Directional mutation pressure, the heterogenicity in the likelihood of different nucleotide substitutions, is used to explain the increasing or decreasing guanine-cytosine content (GC%) in DNA and is represented by microD, in agreement with Sueoka (1962, Proc Natl Acad Sci USA 48:582-592).
The net effect, v/(u + v), has previously been defined as directional mutation pressure (mu D), which explains the wide interspecific variation and narrow intragenomic heterogeneity of DNA G + C content in bacteria.
The parity rules lay the foundation for evaluating the biases in synonymous codon usage in terms of (1) directional mutation pressure for variation of the DNA G + C content due to mutational biases between alpha-bases (A or T) and gamma-bases (G or C), (2) strand-bias mutation, for example, by DNA repair during transcription, and (3) functional selection in evolution, for example, due to tRNA abundance.
The parity rules lay the foundation for evaluating the biases in synonymous codon usage in terms of (1) directional mutation pressure for variation of the DNA G + C content due to mutational biases between alpha-bases (A or T) and gamma-bases (G or C), (2) strand-bias mutation, for example, by DNA repair during transcription, and (3) functional selection in evolution, for example, due to tRNA abundance
Directional mutation theory predicts that when the mutational bias between A/T and G/C nucleotide pairs is equilibrated with the base composition of a neutral set of DNA nucleotides, the mutation frequency per gene will be much lower than the frequency immediately after the mutator mutation takes place

Answer: The rates of substitution mutations in two directions, v (from an AT-pair to a GC-pair) and u (from a GC-pair to an AT-pair), are usually not the same. Thereafter, the effect of mutation on a genome is not random but has a directionality toward higher or lower GC content of DNA. The net effect, v/(u + v), has previously been defined as directional mutation pressure. Thus, directional mutation pressure (GC/AT pressure) refers to mutational biases between alpha-bases (A or T) and gamma-bases (G or C).