Of the three, only the tyrosine residue is important for DNA
melting. It becomes phosphorylated by another domain of the sigma
factor, and the phosphorylated residue mimics a nucleotide
competing with (and displacing) an A residue of the non-template
strand.

These three aromatic residues can stabilize the melted
non-template strand by π−π interactions with the DNA bases in lieu
of the similar interactions formerly afforded by base-stacking
within the dsDNA context.

The three residues are negatively charged and repel the
similarly negatively charged phospo-sugar backbone of the template
strand.

The three residues are positively charged and interact through
ionic interactions with the negatively-charged phospo-sugar
backbone of the DNA non-template strand.