python -regular expression - list element

[antar2]

Hello,

I am a beginner in Python and am not able to use a list element for
regular expression, substitutions.

list1 = [ 'a', 'o' ]
list2 = ['star',  'day', 'work', 'hello']

Suppose that I want to substitute the vowels from list2 that are in
list1, into for example 'u'.
In my substitution, I should use the elements in list1 as a variable.
I thought about:

for x in list1:
   re.compile(x)
	for y in list2:
           re.compile(y)
		if x in y:
			z = re.sub(x, 'u', y)
but this does not work