code: def get_pro_ac_for_tx_ac(self, tx_ac):
          
  
          rows = self._fetchall(self._queries['tx_to_pro'], [tx_ac])
          try:
              return rows[0]['pro_ac']
          except IndexError:
              return None
docs: Return the (single) associated protein accession for a given transcript
          accession, or None if not found.