Laws regarding equity research coverage

What set of SEC guidelines guide equity research analysts on what securities they can and cannot cover? When I do a search on sec.gov, all I get is Spitzer-era guidelines on the relationships of bankers and researchers.

Specifically, I am trying to find out more information about laws in place that either prevent or limit an equity research analyst from initiating coverage on penny stocks, microcaps, or small cap stocks. Thank you in advance to anyone who can shed some light on the topic.