How can I politely inform the person in R&D that MIS are still waiting for a response?
A simple response is best:
MIS still needs a response from HR regarding this issue1 (reference email's title2 + datetime).
I see an email where HR has requested information from Finance1 (reference email's title2 + datetime).
HR are you still waiting on Finance?
Note that my suggested first sentence factually tells the person why you cannot respond yet, then there is a recap.
This is a different order than the answer Snow presented.
Snow also said you should put in the "reason" which isn't a bad idea.
If you do, make sure the email isn't so long that your main point ("we cannot answer yet") isn't lost on someone (most managers) who skims your email.
Often people delete emails, or when the titles aren't the same don't notice that the things are related.
This wraps it all up in one package for them to review - including Finance & HR (one of whom either dropped the ball or failed to give a timely status update).
Note that you aren't blaming anyone. You are just asking politely if HR can proceed, and you have done this in response to a request from you for a status update... it is unlikely anyone will be offended.
Add your manager because you're dealing with other departments (if they aren't already on the chain).
1 I would attach those emails in case someone has deleted them. Especially if you are dealing with managers.
2 If the titles are all the same conversation you can leave this out.