The need for removing the initiator Met in proteins implies the presence of many Ala and Gly at the penultimate amino acid position and consequently many +4G due to the GCN and GGN codons coding for Ala and Gly, respectively
http://plosone.org/article/info:doi/10.1371/journal.pone.0000188#article1.body1.sec1.p3

JW Stewart et al (1971, J Biol Chem 246:7429) proposed that the penultimate amino acid was constrained by the enzymatic requirements for the removal of the initiator methionine by methionine amino peptidase. This is missing in the paper.

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