A. James Johnston, a longtime litigator and employment lawyer at Post & Schell P.C. in Center City, took over Tuesday as the firm's president and chief executive officer. He succeeds Brian Peters, who stepped down in January after leading the firm for 14 years. Johnston is a 1980 graduate of Dickinson College and Pennsylvania State University's Dickinson School of Law, and has chaired the firm's employment and employment relations practice group and its business litigation department. The 135-lawyer firm said that Johnston, while serving as president and CEO, will continue his employment law practice.