code: def transcript_id_of_protein_id(self, protein_id):
          
          results = self._query_transcript_ids(
              "protein_id",
              protein_id,
              feature="CDS")
          if len(results) == 0:
              raise ValueError("Protein ID not found: %s" % protein_id)
          assert len(results) == 1, \
              ("Should have only one transcript ID for a given protein ID, "
               "but found %d: %s" % (len(results), results))
          return results[0]
docs: What is the transcript ID associated with a given protein ID?