You use the tokenat() expression for this, so for example tokenat(cat,0,"") is C, tokenat(cat,1,"") is A, tokenat(cat,2,"") is T. Check the manual for more info!
To have it fill every 5 seconds you would use a variable for the middle value, the index, and every 5 seconds print tokenat(cat,var,"") then add 1 to the variable for next time.