Analysis of 5′ UTR composition and gene expression: Canonical versus non-canonical start codons

The overall composition upstream of start codons in Escherichia coli was evaluated and viewed in connection with global transcriptome data. Genes starting with AUG as initiation codon tended to be expressed at higher levels than the non-AUG genes, and the upstream region of the non-AUG genes showed negligible signs of Shine-Dalgarno sequences. The latter is in sharp contrast to the AUG genes. Viewing these findings in connection with the current literature, it is proposed that a distinct mechanism for initiation of translation might exist for non-AUG genes that are not preceded by a Shine-Dalgarno sequence. A survey covering a range of other eubacteria (Firmicutes, Proteoacteria, and Actinobacteria) reveals that it is mainly among the Proteobacteria that non-AUG genes do not display clear signs of Shine-Dalgarno regions.