High Proportion of Adenines and Thymines in Promoters: Most Likely Reason

The most likely reason promoters of many genes have a high proportion of adenines (A) and thymines (T) is that A-T base pairs are easier to separate than G-C base pairs. This is because A-T pairs are held together by only two hydrogen bonds, whereas G-C pairs have three hydrogen bonds. As a result, DNA regions rich in A and T require less energy to unwind and separate the strands, which is a necessary step for the initiation of transcription by RNA polymerase

Additional Details

• The TATA box, a common element in many promoters, is specifically rich in adenines and thymines. This AT-rich sequence facilitates the binding of transcription factors and the assembly of the transcription initiation complex

• The easier separation of DNA strands at AT-rich regions allows transcription machinery to access the template strand efficiently, promoting the initiation of gene transcription

Summary Table: Base Pairing and Hydrogen Bonds

Base Pair| Number of Hydrogen Bonds| Ease of Strand Separation
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A-T| 2| Easier
G-C| 3| Harder

In essence, the high AT content in promoter regions is an evolutionary adaptation to enable efficient and energy-favorable initiation of transcription.