Learn Turkish: Yeni yönetmelik üzerine kamuoyunun tepkisi büyüdü. - Upon the new regulation, the public's reaction grew.

When üzerine is used to mean "upon" or "as a reaction to" an event or thing, the preceding noun remains in its bare (nominative) form. So yönetmelik üzerine simply means "upon the regulation." If you were using a pronoun instead, it would need the genitive case (like onun üzerine), but regular nouns don't require it in this structure.

Does üzerine just mean "after" here, like -den sonra?

It means "after," but with a strong sense of cause and effect. While yönetmelikten sonra simply means "after the regulation" chronologically, yönetmelik üzerine emphasizes that the public's reaction was a direct and immediate response to the new regulation.

How is the word kamuoyunun formed?

The root is kamuoyu (public opinion / the public). This is a fixed compound made of kamu (public) + oy (vote/opinion) + the possessive suffix -u. To say "the public's reaction," we need the genitive case, so we add the buffer consonant -n- and the suffix -un, resulting in kamuoyunun.

Why is it tepkisi instead of just tepki?