def nucl_count(sequence):
    """determine counts of each letter in a sequence,
    return dictionary with counts"""
    nucl = {'A': 0, 'C': 0, 'G':  0, 'T': 0, 'N': 0}

    for letter in sequence:
        if letter not in nucl:
            print(f'unexpected nucleotide found: "{letter}"')
            nucl[letter] = 0
        nucl[letter] += 1

    return nucl