The process of translation initiation in E. coli is summarized in the figures below. It is complete when the ribosome has formed a complex with the mRNA, and the first tRNA, which is always charged with f-met in E. coli, is hydrogen-bonded to the start codon. In E. coli, the start codon is the first AUG or GUG downstream from the Shine-Dalgarno sequence which is a short region with a consensus sequence of AGGAGGU. The start codon is usually about 6-8 nucleotides downstream from this. Therefore, it is the interaction between the Shine-Dalgarno sequence on the mRNA and the complementary sequence on the 16S rRNA that really "defines" what codon will be the start codon.

A video of this process:

Prokaryotic Translation

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