????by Jessica Shambora

????Yesterday on Postcards we wrote about an historic event: Xerox (XRX) CEO Anne Mulcahy announced she will pass the baton to president Ursula Burns on the first of July in the first ever woman-to-woman transfer of the CEO role in the Fortune 500. Burns will also be the first African-American female to lead a company on the list. (Click here for a 2006 Fortune profile of Burns.)

????While Fortune, in 2003, called Mulcahy “The Accidental CEO,” Burns, 50, is an equally unlikely chief executive. She grew up up in a housing project on Manhattan’s lower east side and was raised by a single mother. Her mom scraped together enough funds from doing domestic work to send her daughter to Catholic school, then Columbia University. Burns studied engineering there.

????Burns talked candidly about her mother on stage at a Fortune Most Powerful Women dinner in Manhattan last spring– exactly one year ago today. “I’ve had many mentors, but the one that has the most impact was my mother,” she said. Burns has a sign on her office wall: “Don’t do anything that wouldn’t make your Mom proud!”

????Burns’ mother died 25 years ago. But in a recent commencement speech at Worcester Polytechnic Institute, Burns told the grads that her mom “is perched on my shoulder whispering encouragement and keeping me honest, keeping me humble.”

????Indeed, humility has been critical to pulling off Xerox’s turnaround. Burns, who began her Xerox career as a summer intern in 1980, smartly recognized that she needed to broaden her skills to become a full-fledged leader. And who better to help her develop those skills than Mulcahy? Fortune’s 2007 cover story, “Xerox’s Dynamic Duo,” explored the delicate power-sharing between the two women. Hard-charging and blunt, Burns admitted to “letting my big mouth drive the discussion” and conceded, “Patience is not one of my strengths.”

????So Mulcahy helped coach her in both areas. Says Burns: “Anne has taught me that 90% is getting the rest of the organization to line up. Anne is a master at that.”

????Burns ascended through engineering and product development — sharing her expertise in those areas with Mulcahy, who shared her own skills in sales and HR. When Mulcahy became CEO in August 2001, she tapped Burns to lead manufacturing, which happened to be one of the toughest jobs in the company. As Burns was renegotiating a contract with 2,000 unionized workers in Rochester, N.Y., she was simultaneously exploring sales and closures of the facilities where these people worked. Burns was also recovering from a hysterectomy. She held talks from her hospital bed and later from her living room.

????The mother of two children (a sophomore at MIT and a junior in high school), Burns is also on the boards of American Express (AXP) and Boston Scientific (BSX). At Xerox, she’s taking the helm at another challenging juncture. Demand for printers is weak; the strong U.S. dollar has been squeezing profits. Xerox stock is down 50% in the past year. It’s trading below $7.

????Fortunately, Burns is used to making the best of a tough situation. She told the Worcester Poly grads last weekend, “This old notion that work is drudgery is nonsense. Most days, even back when Xerox was under siege, I could not wait to get to the office. I love my work. And you should too.”