Karen K. Johnson, a convicted drug dealer who was jailed three years ago for refusing to testify about possible drug use by District officials and others, is cooperating with federal investigators probing allegations that she received payments in exchange for her silence, sources said yesterday.

In at least four long meetings with federal prosecutors, Johnson has answered questions about alleged payments from D.C. businessman John B. Clyburn, whose firms have held several contracts with the city, and about her prior personal relationship with D.C. Mayor Marion Barry, the sources said.

During a 17-month undercover investigation into D.C. countracting, FBI agents, using wiretapping and surveillance, uncovered evidence of sporadic payments from Clyburn, the sources said. Federal authorities also are investigating whether Roy Littlejohn, another prominent D.C. contractor, also paid Johnson, the sources said. Clyburn and Littlejohn have denied any wrongdoing.

Barry, reached in Nashville yesterday where he is attending a meeting of the U.S. Conference of Mayors, said he was pleased Johnson was talking to prosecutors. "I'm glad," he said. "I hope she tells them everything and tells them the truth. It helps me immensely. Put that on the top of the story."

Barry said last month he "has not talked to {Johnson}, or had anybody else to talk to her or contact her in any way" for 4 1/2 years.

Sources characterized the cooperation of Johnson as a significant breakthrough for prosecutors in the investigation of possible obstruction of justice by Clyburn and others.

Clyburn is also a key figure in a larger racketeering investigation that centers on allegations that top D.C. officials steered contracts to selected firms.

Johnson, 35, declined to comment yesterday on her meetings with prosecutors.

Barry said, "If there were any payments, I don't know anything about that. Absolutely, unequivocally."

"If there was anything connecting me with any of this nonsense, {federal authorities} would have made sure . . . it was leaked to you," Barry said. "That's their tactics. I am tired of people trying to link me with some alleged payoff."

Barry previously has described Clyburn as a "good political friend" and, in a May 13 deposition filed in a separate civil suit, called Clyburn "a very close personal friend of mine" whom he relies on for advice "about a range of activities involved in the city."

Johnson was sentenced to 18 months in jail in 1983 after pleading guilty to cocaine conspiracy and possession charges, with all but four months suspended. While serving the four-month sentence in a halfway house, she was held in contempt for refusing to testify before the grand jury about possible drug use by D.C. officials, including Barry, and others, and was jailed for eight months. She subsequently completed her sentence on the cocaine charges in the halfway house.

Johnson, who was released in July 1985 and remains on probation until next month, would be required to serve the other 14 months of her original sentence if a judge determined she had violated her probation.

G. Allen Dale, Johnson's attorney, said yesterday that Johnson had not committed any criminal acts or done anything else that "would constitute a violation of probation." No charges have been filed against Johnson or anyone else in connection with the investigation.

The FBI began investigating activities by Clyburn last year after the president of a D.C. asbestos firm told prosecutors that a Detroit-area businessman had attempted to extort money from him in return for help in obtaining two multimillion-dollar contracts. Using an undercover agent posing as a businessman, FBI agents were able to gather enough evidence to obtain authorization for wiretaps. Subsequently, one source said, federal authorities learned of possible payments to Johnson.

Investigators have been trying to determine why the alleged payments were made, when they began and whether Clyburn made them on his own behalf or on behalf of others. During a May 22 search of Johnson's Northwest row house, FBI agents seized bank statements and other financial records.