Beamer - Make "alert" font highlighted

In the Beamer class, How can I make the alert font highlighted as in hl?
Then, how can I make it so a word is alerted (highlighted) for only one slide?
i.e., I want this to be the output:
Slide 1: ${x^{alert{2}} - x - 1}$
Slide 2: ${x^{2} - x - alert{1}}$
Slide 3: ${x^{2} - x - 1}$

This is the code I added in order to get hl to work in Beamer:

makeatletter
letHLhl
renewcommandhl{
  letset@colorbeamerorig@set@color
  letreset@colorbeamerorig@reset@color
  HL}
makeatother