Finding a Protein Motif
When cracking argv, you have an opportunity to use argparse if you wish.

Please bury all this code under def main():, rather than creating lots of top-level globals like f.

Rather than f = open() ... f.close(), you might phrase it this way:

with open('ros.fa', 'w') as f:
    ...


There are some comments you might elide. The identifier string is accurate, but a bit vague. Might be better to elide the temp variables:

f.write(req.data.decode('utf-8'))


Overall, this code looks like it gets the job done.