Now that the decision has been made, though, the city's focus needs to be on "back-filling the vacancy created [at Constellation's current headquarters on East Pratt Street and next door at the Candler Building on Market Place] as quickly as humanly possible," Cole said.

Several hundred thousand square feet of office space along Pratt Street may depress office lease prices uptown, said Terri Harrington, a commercial real estate broker with MacKenzie Commercial Real Estate Services in Baltimore. As more space comes on the market along Pratt Street and prices drop, she said, tenants north of Pratt who previously couldn't afford prime space in the central business district will move, leaving less desirable spaces empty.

Recent moves into downtown — such as Transamerica Life Insurance Co. moving into Legg Mason's former home at 100 Light St. — provide reason for optimism, said Ryan O'Doherty, spokesman for Mayor Stephanie Rawlings-Blake. Legg Mason moved its employees to a new structure in Harbor East in 2009.

"Recent history has proven beyond a doubt that it is possible to have growth in Harbor East and in a downtown building" that a company vacated, just as is happening with Constellation, he said. "Harbor East versus downtown doesn't need to be an either/or situation."

Rawlings-Blake also supports converting office buildings into residential space if there is an appetite for it, O'Doherty said. Baltimore Gas and Electric Co.'s old building on East Lexington Street, for instance, is now a thriving apartment structure, he noted.

City Councilman James B. Kraft, whose district includes the Harbor Point development, said the area has been zoned for dense commercial development for more than a decade and can absorb the influx of human and vehicular traffic that will result from Exelon's arrival. He expects temporary "growing pains" as Harbor East and Fells Point adapt to more bodies and cars, he said.

But Robert Manekin, managing director and principal at the commercial brokerage firm Colliers International in Baltimore, was more worried. "There is a very real concern in the marketplace that the density cannot be effectively addressed by the current streets and mass transit," he said.

If the merger is completed, Exelon would maintain its headquarters in Chicago, but the Baltimore office would house its growing power-selling and renewable energy businesses. About 1,500 workers are now based at Constellation's downtown headquarters.

The merger would lead to the elimination of about 600 positions across both companies, but the job reductions would be felt most in Baltimore, affecting Constellation's legal, information technology, financial and other corporate departments, according to Exelon officials.

Exelon plans to move 200 to 250 jobs from its Pennsylvania energy-trading operations to Baltimore.

Exelon put out a request for proposals on a headquarters site in August and by late October had focused on proposals from nine developers. The company narrowed the field even further and received input from "key community stakeholders," Exelon said.

Michael S. Beatty, president of Harbor East Development, said in a statement that Exelon wanted a "unique headquarters facility that would create a dynamic home for their growing Baltimore workforce."

"This project will be a model catalyst for the development of a truly sustainable and vibrant neighborhood," said Beatty, whose group also developed the Thames Street Wharf building anchored by Morgan Stanley. In addition to being involved in waterfront development, Paterakis heads the nearby H&S Bakery Inc.

Chromium chemical processing took place on the Harbor Point site for more than 100 years, according to the Maryland Department of the Environment, and required a decade of cleanup to make the acreage suitable for construction. The environmental remediation was completed in 1999, leaving behind a hydraulic barrier to prevent chromium from leaching into the harbor and a 5-foot-thick protective cap that stops the dangerous substance from being released on the surface of Harbor Point's property.

Exelon said it has selected Washington D.C.-based SmithGroupJJR as the interior design architect; Baltimore-based RTKL as the mechanical, electrical and plumbing engineer, and GreenShape in Washington D.C. as the consultant for Leadership in Energy and Environmental Design.

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