What features of the structure of mRNA enable its interaction with the 30S subunit?

Toward the 5′ end of mRNA there is a region of ~20 nucleotides prior to the initiation codon AUG. This leader region contains a purine-rich sequence that is responsible for the interaction of the mRNA with the 30S subunit. It is known as the Shine-Dalgarno sequence (after John Shine and Lynn Dalgarno, the Australians who made the discovery); it can
bind to a complementary sequence at the 3′ end of the 16S rRNA to orient the 30S subunit appropriately for initiation.

The Shine-Dalgarno sequence distinguishes the initiating AUG, which also determines the reading frame, from an AUG that encodes an internal methionine. Other sequences in the leader region are possibly involved in the overall process of initiation of translation, which also involves the binding of the appropriately charged methionyl-tRNA opposite the AUG codon.