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
.