Next he moved to Rome , where he worked for the Grand Duke of Tuscany , who had a large household there . In 1553 , he became maestro di cappella of the Basilica di San Giovanni in Laterano in Rome . Next he moved to Rome , where he worked for Cosimo I de ' Medici , Grand Duke of Tuscany , who maintained a household there ; and in 1553 , he became maestro di cappella of the Basilica di San Giovanni in Laterano , the ecumenical mother church of Rome and a spectacularly prestigious post indeed for a man only twenty-one years old .