*3.4. uATGs—Flanking Sequence Context*

The median value of uATG flanking sequence context in our study was 2 (on a scale of 1–4), minimum 1 and maximum 3. The median value of sATG flanking sequence context was 3, minimum 1 and maximum 3. Churbanov et al. [31] wrote that there was no significant difference between the nucleotide contexts of the conserved and non-conserved uATGs; in both cases, the information content of the uATG context was lower than that of the sATG context. Similarly, the analysis of the oligonucleotide context of uATGs, uORFs and sATGs has shown that a significant preference bias can be observed only for sATGs which, on average, have a much better context than uATGs and uORFs [32]. Our results are also in agreement with Rogozin et al. [23], who found that the presence of ATG triplets in 5′UTR regions of eukaryotic cDNAs correlates with "weaker" contexts of the sATGs. The median sATG context in our study was "strong" (level 3), not "optimal" (level 4). However, newer results on a larger cohorts of annotated transcripts have indicated some specifications, e.g., the proportion of uATGs in optimal contexts for conserved uORFs was noticeably higher than for non-conserved uORFs, while still half the proportion for main ORFs [33].
