def get_pro_ac_for_tx_ac(self, tx_ac):
rows = self._fetchall(self._queries['<STR_LIT>'], [tx_ac])<EOL>try:<EOL><INDENT>return rows[<NUM_LIT:0>]['<STR_LIT>']<EOL><DEDENT>except IndexError:<EOL><INDENT>return None<EOL><DEDENT>
Return the (single) associated protein accession for a given transcript
        accession, or None if not found.