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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def aa3_to_aa1(seq):
 """convert string of 3-letter amino acids to 1-letter amino acids 'CATSARELAME' """

    if seq is None:
        return None
    return "".join(aa3_to_aa1_lut[aa3]
                   for aa3 in [seq[i:i + 3] for i in range(0, len(seq), 3)])