Bloomberg took another step Thursday toward expanding its media empire, announcing plans to buy an Arlington County publisher of legal, tax, business and government news services for $990 million.

The deal for the Bureau of National Affairs is the largest in Bloomberg’s 30-year history and comes as the New York-based financial news and data juggernaut seeks to build franchises focused on the legal and government markets.

With more than 600 reporters, lawyers and editors on staff, BNA, founded in 1929, offers a wealth of legal, legislative and regulatory news and commentary through about 300 online and print outlets. The company, which reported 2010 revenue of $331 million, is known for such publications as U.S. Law Week and Daily Labor Report.

BNA will operate as a stand-alone subsidiary, although it will work in tandem with Bloomberg’s current offerings to “deliver more premium content,” Bloomberg chief executive Dan Doctoroff said in a statement announcing the deal.

The company declined further comment.

Bloomberg’s move comes as a growing list of media outlets target speciality news services in an effort to cater to niche audi­ences. In October, the political news site Politico, for instance, introduced Politico Pro, a subscription-based service providing micro-level coverage of energy, technology and health care. “Everyone is looking around for additional sources of revenue that are in anyway complementary to what they’re doing,” said Michael E. Parker, managing director at media investment bank AdMedia. “Bloomberg is making a considerable investment toward future revenue sources.”

The BNA acquisition follows a series of moves that have upped Bloomberg’s profile in the government and legal data arenas. Early last year, the media powerhouse laid claim to Eagle Eye Publishing, a provider of data on federal procurement.

Bloomberg used that acquisition to help start Bloomberg Government, or, focusing on government policy and regulatory news and data. For the new venture, the company leased new office space in the District and added more than 100 people to its payroll.

The company has been trying to build a similar brand around Bloomberg Law.

“By itself, this acquisition is not a short-term game-changer in legal information, but it does confirm Bloomberg’s intention to become a more aggressive competitor in the legal space,” Barclay Capital analyst Paul Sullivan said in a note discussing the deal.

As part of the BNA transaction, slated to close before the end of the year, Bloomberg will acquire all outstanding shares of the company for $39.50 per share in a cash tender offer to commence Sept. 8. Considering that BNA is wholly owned by its employees, that offer may result in a hefty payday for longtime staff members.

The deal also pairs Bloomberg’s non-unionized workforce with BNA’s unionized one. Officials at Bloomberg said the new subsidiary will remain unionized.