Possible identity of transcription and translation signals in early vital systems

The distribution of codons was analysed in three classes of eukaryote proteins having widely different evolutionary rates: 78 histones, 40 tubulins, and seven fibrinogens. In this set of genes, (i) it was confirmed that codons which are components of known transcription signals, like ATA, are used infrequently when a synonym is available, particularly in the more constrained proteins, and (ii) it was observed that the three codons which have an iso-accepting transfer with anticodon UAA, UAG or UGA are also suppressed. Then, the distribution of UAA, UAG and UGA trimers was studied in 498 tDNAs and 198 rDNAs. It was found that these trimers are weakly but significantly suppressed in tDNAs and to a lesser extent in rDNAs. It was advanced that the present suppression of ATA, which codes for Methionine in several mitochondria, and of the TAA, TAG and TGA trimers in tDNAs, might be an indication that at the very early stages of the evolution of translation and transcription the signals for initiation and termination were shared by the two processes.