It varies by Megacorp. In most cases, Megacorps are basically the "real" government as it relates to normal, everyday life but national governments techincally remain (Ares in UCAS, Saeder Krupp in Germany, etc.) but occasionally the regional megacorp has literally taken over the country, often in the case of the more "evil" megacorps in regions lacking a Great Drgaon (most notably Aztechnology in Mexico).