Clippy from the Rust tool chain has a hint for optimising the code:

2 |     dna.replace("T", "U")
  |                 ^^^ help: try using a `char` instead: "T"
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#single_char_pattern
  = note: `#[warn(clippy::single_char_pattern)]` on by default