CNN announced Thursday that Tom Johnson will step down as the network’s chief. Johnson, who held the post for 10 years, will remain as an advisor and consultant. The news comes just months after CNN and other Turner Broadcasting Network channels became a single unit consisting of the television properties at AOL Time Warner. Jamie Kellner, who was named head of the new AOL Time Warner TV unit in March, planned to turn CNN around after ratings dropped amid competition from Fox News Channel and MSNBC. Kellner was also behind the move to bring Moneyline anchor Lou Dobbs back and approved the hiring of former NYPD Blue star Andrea Thompson as an anchor for Headline News.