The prevention of a nuclear Iran constitutes a top US national security priority. It sheds light on a special aspect of US-Israel relationship: defiance of mutual threats.
Iran pursues nuclear capabilities to advance strategic goals, which are led by the super-goal: hegemony over the Persian Gulf and its natural resources. Those who undermine the super-goal are considered super-enemies, targeted by super-capabilities. Hence, Teheran would use its nuclear power/threat, first and foremost, to force the US and NATO out of the Gulf and the Indian Ocean. It would then turn it against Iraq - its arch rival since the seventh century - and against Saudi Arabia, which is considered an apostate regime. All Gulf states are perceived by Iran as a key prize, required in order to control the flow and the price of oil and to bankroll its megalomaniac regional and global aspirations (e.g. leading Islam's drive to dominate the globe).
The Jewish state constitutes a non-Gulf basin target for Iran, not a primary target. Moreover, Israel is expected to retaliate in a traumatic manner, which would paralyze much of Iran's military and civilian infrastructure. Therefore, Iran would not sacrifice its super-goal (forcing the US out of the Gulf and subjugating the Gulf states) on the altar of a secondary-goal (obliterating the Jewish state).