We wouldn't write the code for you from scratch (thats your job), but its an easy problem because in Python any string is an "array" or "dictionary". Thus 

    bacteria1 = 'GRRKAAVARAYMTAGKGNVTVNKKDYKDFFPVPTLQYKITQPFAITETLGQYDVIEEXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX'
    print (bacteria1[0:120])

This will define the first gene in your list, remembering the array starts at 0 whilst your gene starts at position 1 (so you need to subtract 1). You simply parse the input list so the gene lengths form a dictionary and loop through the dictionary of gene lengths ensuring the last length is added to the current one and then issuing a write command. 

You don't really need biopython in this instance, unless it will parse the gene length file you need to open (I'm not aware it can).

-----

BTW the format you supplied isn't fasta it looks like a Biopython statement, again in this problem I wouldn't bother because Python is perfectly capable of splicing a string.