Shorenstein Properties, a prominent San Francisco-based developer, is negotiating to buy the 44-story Exxon Mobil building in downtown Houston in what could become a catalyst for redevelopment in the southern end of downtown.

Shorenstein recently reaffirmed its expertise in reworking classic office space with its acclaimed redevelopment of the historic Market Square building in San Francisco for the headquarters of the Twitter social media company.

Exxon Mobil is selling its aging 1.1 million-square-foot downtown tower in conjunction with its relocation to its huge new corporate campus, which is under construction in Houston’s northern suburbs.

Shorenstein declined to comment on the deal, but sources in the real estate community said the San Francisco developer would place the Exxon Mobil tower in one of its powerhouse investment funds — Shorenstein Realty Investors Ten, which is capitalized with $1.23 billion.

The Exxon Mobil building, which opened in 1963, is viewed by some as an outdated property. The tower lacks a connection to the downtown tunnel system, a flaw that prevents it from being on the top rung of preferred office locations. Surrounded mostly by surface parking lots on the southern end of downtown, its location at 800 Bell is a bit removed from the prime office addresses on Louisiana or Smith Streets or the Houston Center complex.

Originally known as the Humble Oil Building, it was the tallest building west of the Mississippi when it opened 50 years ago. Located on the top of floors of the building, the renowned Petroleum Club has been the venue of countless business lunches and banquets.

If the sale proceeds to closing, Shorenstein is expected to pay around $50 million for the Exxon Mobil building. As part of the deal, Exxon Mobil would lease back the tower for a couple of years until its new campus is completed.

The $50 million purchase price is modest for a Houston skyscraper today. The Shell Plaza in downtown recently sold for around $550 million and 64-story Williams Tower in the Galleria area is expected to fetch about $500 million.

However, office rents in downtown Houston are exceptionally strong right now, so Shorenstein is in line to reap major returns on its investment.

With the Market Square building in San Francisco, where Twitter occupies over 220,000 square feet, Shorenstein recently took a dilapidated Art Deco building constructed in 1937 (the same year the Golden Gate Bridge was built) and transformed it into a modern showcase work space.

CNBC, the business news cable channel, listed the Twitter offices as one of the “coolest corporate headquarters” in America. Shorenstein’s renovation of the Twitter building, located on San Francisco’s Market Street, also was viewed as transformative for a rundown stretch of the Mid Market neighborhood.

Shorenstein’s investment programs are typically focused on “Class A” office properties – the prime buildings that get the highest rents. So buying the less-than-prime Exxon building offers a clue that Shorenstein envisions a major redevelopment of the tower.

Shorenstein owns many office buildings across the nation including two in Houston near the intersection of San Felipe and Loop 610: the 28-story Five Post Oak Park building, which is across the street from the St. Regis Hotel, as well as the 21-story 2000 West Loop South tower on the freeway’s west side frontage road.