By Matt Smith DUBAI, April 22 (Reuters) - Saudi Arabia plans to regulate local companies that screen shows on YouTube, a senior official was quoted as saying in local media on Tuesday - a move that could stifle its nascent creative industries. The kingdom is the world's top per capita user of YouTube. But YouTube's popularity has **w brought Saudi Arabia's homegrown production houses under the gaze of the General Authority for Audiovisual Media, a recently formed watchdog. Riyadh Najm, the Authority's president, told the pan-Arab newspaper Asharq al-Awsat that his organization would soon issue a manifesto to organize - or regulate - the work of YouTube channels.