Does PNAS allow authors to request referees who have not co-authored with them but work at the same university? I'm looking to request an academic who works in an entirely different department and is an expert in the field, but he is in the same university and we have yet to co-author a paper together. Do you know if PNAS allows that? Is it too ethically ambiguous?
Though I have a letter published in PNAS, I don't know PNAS policy on recommending reviewers.  I suggest the following:


Recommend this professor/researcher, making clear that he/she is at your university and in a different department.
Make clear that you've have not co-authored a paper together and have no other common interests (e.g. funding).
Explicitly tell them that this is a potential conflict of interest in the reviewer role (bias in preference of the same university), and that you trust PNAS to make a judgment in this particular case.
Make other recommendations for reviewers so that the PNAS editors have other choices if they aren't comfortable with the controversial recommendation.


The main principle in these cases is to be open, forthcoming, and transparent.  Even if the PNAS editors do not take your recommendations, you shouldn't be hurt by making them.