Remap Ctrl-A in GNU Screen

To make it CTRL+J, use the following in .screenrc.

escape ^Jj

To quote the manpage,

 escape xy

Set the command character to x and the character generating a literal command character (by triggering the "meta" command) to y (similar to the -e option). Each argument is either a single character, a two-character sequence of the form "^x" (meaning "C-x"), a backslash followed by an octal number (specifying the ASCII code of the character), or a backslash followed by a second character, such as "\^" or "\". The default is "^Aa".

Similar to the solution of @Alex Hirzel but I prefer using keybinding Ctrl+^ (Ctrl+Shift+6) as it is never used in emacs keybindings.

Just write in ~/.screenrc as follows:

escape ^^^