MediaNews Group Inc., a privately held Dallas firm that has become one of the country's fastest growing newspaper chains, yesterday boosted its holdings by buying the Houston Post from Toronto Sun Publishing Co. for $150 million.

Coming on top of the firm's purchase last year of the Dallas Times Herald, MediaNews Group's acquisition of the Post makes it one of the more formidable media voices in Texas.

The purchase also gives new prominence to the company's 35-year-old president, William Dean Singleton, who has made no secret of his interest in solidifying his native Texas base. Last March, Singleton bid $415 million for the Houston Chronicle, the city's larger newspaper, which was sold instead to the Hearst Corp.

John Reidy, a media analyst with Drexel Burnham Lambert Inc., said the Post purchase reinforces Singleton's reputation in the industry as a "very aggressive entrepreneur." But, he added, his ultimate success will hinge in large part on the future of the depressed Texas economy.

Asked whether the Post is likely to be a profitable purchase, Reidy replied, "It all could boil down to what the price of oil is going to be. He could really look like a hero if the price of oil goes through the roof."

Within a week after taking over the money-losing Dallas Morning Herald from the Times Mirror Co. last September, Singleton moved to slash costs, including the layoffs of 109 employes. But Singleton said yesterday he had no such plans for the Houston Post, in part because, unlike the Morning Herald a year ago, the Houston paper is profitable.

Singleton also said he will leave in place the Post management team, including publisher Donald F. Hunt.

"Don Hunt and his management team have met the challenge of operating profitably in the tough Texas economy while showing impressive growth both in advertising market share and circulation," said Singleton in a prepared statement.

According to the Audit Bureau of Circulation, the latest circulation for the morning Post for the period ending March 31 was 309,400 daily and 356,424 on Sundays. For the same period, the afternoon Chronicle's circulation was 406,084 daily and 531,528 on Sundays.

The Toronto Sun firm, which is in turn owned by media conglomerate MacLean Hunter Ltd., bought the Post in December 1983 for $91.7 million from a communications firm owned by Oveta Culp Hobby. Sun Publishing President Doug Creighton said yesterday in a statement that the company "had met all its original goals in Houston to rejuvenate the product."

Under the terms of the deal, Sun Publishing will continue to have a stake in the Post's operations. MediaNews' Gloucester County Times Inc. affiliate, which is formally acquiring the newspaper, will pay Sun Publishing $150 million in cash immediately. Sun Publishing will receive an additional amount on March 31, 1993, based on the gross revenues of the Post during the five years ending Dec. 31, 1992.

With the purchase of the Post, MediaNews Group will own 27 daily newspapers and 28 non-dailies, with its largest holdings in New Jersey and California,as well as in Texas. The firm is owned as a partnership of Singleton and Richard B. Scudder, whose family has long owned newspapers in New Jersey.