Simplify the following text for a Grade 12 native English-speaking person.
Original: He is general director and artistic director of the Mariinsky Theatre , principal conductor of the London Symphony Orchestra , and artistic director of the White Nights Festival in St. Petersburg .
Simplified: He is general director and artistic director of the Mariinsky Theatre , principal conductor of the London Symphony Orchestra and principal guest conductor of the Metropolitan Opera , New York . Valery Gergiev is the artistic director of the White Nights Festival in St. Petersburg .