Nucleotide Usage in S. cerevisiae

The prepared input sequences contain a total of 8,553,510 bases, with an overall CG content of 39.6%. Figure 1 summarizes the proportion of nucleotides appearing in each intracodon position. Almost one third of all nucleotides are adenosine (A), which is found in the first and second intracodon position more often than any other nucleotide. In the third codon position adenosine is slightly less abundant than thymidine (T), which ranks second overall. The other nucleotides are also distributed asymmetrically in the three intracodon positions. The rank order is AGTC in the first position, ATCG for the second position, TACG for the third position, and ATGC overall.

Figure 2 shows the distibution of nucleotides by position. Guanosine (G) shows a strong bias for the first position, where 46% of all guanosines occur, and against the second position, where only 23% occur. In contrast, adenosine is more evenly distributed over the three positions.