The small subunit of ribosome select the initial AUG codon of mRNA to be placed at its P site as opposed to some internal one in bacterial ribosome  because of 

Bacterial mRNAs possess a specific sequence of nucleotides (called the Shine‐Dalgarno sequence after its discoverers) that resides five to 10 nucleotides before the initiation codon. The Shine‐Dalgarno sequence is complementary to a sequence of nucleotides near the 3 ′ end of the 16S ribosomal RNA of the small ribosomal subunit.