CTRL-A is used in emacs to move the cursor to the beginning of the line. It can also be used on the bash command line if emacs-style line editing is turned on with set -o emacs. It's use is so automatic and thoroughly ingrained that 'screen' would seem to be almost worthless to emacs users.
But fortunately, we can add a line to the screen configuration file: ~/.screenrc:
That is, the keyword "escape" followed by two backticks. That way, we use the backtick (the unshifted tilde key) instead of CTRL-A. That could interfere with shell programming, but it's more readable to use "$()" instead of backticks anyway. And if you really want to type a backtick, just hit the key twice.