def bitstring_to_bytes(bitstr):
    """
    Converts a pyasn1 univ.BitString instance to byte sequence of type 'bytes'.
    The bit string is interpreted big-endian and is left-padded with 0 bits to form a multiple of 8.
    """
    bitlist = list(bitstr)
    bits_missing = (8 - len(bitlist) % 8) % 8
    bitlist = [0] * bits_missing + bitlist
    result = bytearray()
    for i in range(0, len(bitlist), 8):
        byte = 0
        for j in range(8):
            byte = byte << 1 | bitlist[i + j]
        result.append(byte)
    return bytes(result)
