You have two options with simple and idiomatic Typescript:
DNATranscriber: { [char: string]: string } = {
G: "C",
C: "G",
T: "A",
A: "U",
};
This is the index signature the error message is talking about. Reference
DNATranscriber: { G: string; C: string; T: string; A: string } = {
G: "C",
C: "G",
T: "A",
A: "U",
};