Well, it really wasn't meant for golf originally ... if we're going to do that, I propose ...

51 characters

Last@SubstitutionSystem[{"x"->"{x}_{x}^{x}"},"x",5]

48 characters

Nest[StringReplace[#,"x"->"{x}^{x}_{x}"]&,"x",5]

45 characters

Nest[StringReplace["x"->"{x}^{x}_{x}"],"x",5]

P.S. It's a pity that TeXForm does not support Subsuperscript.

40 characters

Nest["{"<>#<>"_"<>#<>"^"<>#<>"}"&,"x",5]

Didn't know about SubstitutionSystem!

40 characters

Reformulating {}-bracketing in last Szabolcs solution cuts 5 characters from it:

Nest[StringReplace["x"->"{x^x_x}"],"x",5]