I wasn’t aware US companies were doing business with Iran.
The whole concept of the Iranian government ‘sanctioning’ American companies is ludicrous since it is they who are under existing sanctions. This amounts to playground name-calling. Par-for-course for the Mullahs.
After the Iran nuke deal, U.S. companies with foreign entities could do business with Iran through the foreign offices. Also, those dealing in medicine, healthcare, for humanitarian reasons - can do limited business there.
Being signed a deal to sell Iran 80 planes at the end of 2016, but it’s not certain it will be able to go thru with it now. Hopefully, Trump’s sanctions will stop it. Though they’re getting planes from Airbus.