Less than a year ago, federal markets regulator Gary Gensler could reflect fondly on his association with Wall Street mogul and former politician Jon S. Corzine.

The two had known each other for years, working together at Goldman Sachs and then in Congress, where Corzine was a senator and Gensler was a Senate aide. Invited by Corzine last November to speak at Princeton University about financial regulation, Gensler reminisced about their days crafting legislation and got in a friendly jab about Corzine’s upcoming wedding.

“Jon, your life has changed a lot since our days together on a trading floor if this is your idea of a bachelor party,” Gensler said.

Now, the relationship has taken on an entirely different cast. Gensler, as chief of the Commodity Futures Trading Commission (CFTC),  is one of the key regulators probing the brokerage firm MF Global that Corzine led into bankruptcy.

Corzine resigned Friday as head of MF Global, but he had already lost control of the firm to a trustee.

According to regulators, hundreds of millions of dollars are missing from customers’ accounts, and it falls largely to Gensler’s agency to figure out what happened. The FBI and other authorities also are investigating.

For Gensler, the collapse of MF Global poses a variety of questions. How could it have happened? Did the CFTC intervene too abruptly this week when it helped put the firm under the control of an independent trustee — or not soon enough?

And, given his past association with Corzine, should Gensler play any role in the probe?

The meltdown at MF Global is another in a series of challenges for Gensler, who has undergone one of the more dramatic transformations in the regulatory arena.

During the Clinton administration, he and other administration officials resisted efforts to regulate the complex instruments known as derivatives, which later contributed to the financial crisis. Now, Gensler is forcefully leading efforts to impose restraints on the industry.

MF Global’s bankruptcy could give him new momentum, or it could become a troubling diversion.

In a statement, Sen. Charles E. Grassley (R-Iowa) said that, for the sake of investors, it seems Gensler should recuse himself from the MF Global case.

“It’s hard to see how the commission chairman could be completely objective in looking out for wronged investors when he has such strong ties to the principal of the failed firm,” Grassley said.

At a hearing this week, Sen. Tom Coburn (R-Okla.) jousted with Gensler over regulatory policy and then raised the issue of MF Global.

“I’m very interested in how in the world did that get past you all,” Coburn asked.

Gensler had tried to increase protections for brokerage customers.

Last year, the CFTC proposed tightening restrictions on what brokerage firms can do with customers’ funds. But the proposal met opposition from members of the industry, including Corzine. MF Global wanted the freedom to continue engaging in transactions by which it can essentially borrow funds from customers’ accounts.

Corzine and members of his staff discussed the subject with Gensler and members of his staff in a July 20 conference call, according to CFTC records.

Amid the industry opposition, the CFTC has yet to adopt the proposal. Commissioner Bart Chilton said Gensler never wavered in his support for the measure but didn’t have the votes to get it passed.

Rep. Barney Frank (Mass.), the top Democrat on the House Financial Services Committee, said it was far too early to be asking Gensler to formally step away from any investigation.

“Knowing somebody is way too broad a reason for recusal,” he said.

While some observers have criticized the close relationships between Wall Street firms such as Goldman Sachs and the regulators supposed to oversee them, Frank said someone with with no connections “might have no knowledge.”

Gensler was born in Baltimore and attended the University of Pennsylvania’s Wharton School of Business, where he earned a BS in economics and a master’s in business administration.

He joined Goldman Sachs in 1979 and stayed for 18 years before leaving for the Treasury Department during the Clinton administration. There he was assistant secretary under another Goldman alum, Secretary Robert Rubin, and later, Secretary Lawrence H. Summers.

He is also the co-author of a book called “The Great Mutual Fund Trap: How Americans Are Losing Billions to the Mutual Fund and Brokerage Industries — and How You Can Earn More with Less Risk.”

After the Clinton administration, Gensler worked as an adviser to then-Senate Banking Committee Chairman Paul Sarbanes on the Sarbanes-Oxley Act, a corporate accounting reform bill. It was his collaboration with Corzine during that time that was the basis of the reminiscence he laid out last year for the Princeton crowd:

“He offered the first bill in the Senate Banking Committee to reform accounting oversight after Enron collapsed, and he played a significant role in what later became the Sarbanes-Oxley Act,” Gensler said, according to a text of the speech on the CFTC Web site. “In fact, when the full Senate voted on the bill, if memory serves, Jon was sitting in the presiding chair and I was staffing Chairman Sarbanes on the floor.”

Gensler’s 2008 nomination to head the CFTC languished for months because some members of the Senate doubted that he would be a tough-enough regulator. At his confirmation hearing, Sen. Tom Harkin (D-Iowa) cited Gensler’s testimony years earlier in opposition to greater regulation of certain derivatives.

“Looking back now,” Gensler said, “it is clear to me that all of us that were involved at the time, and certainly myself, should have done more to protect the American public through aggressive regulation, comprehensive regulation.”

Since becoming chairman, Gensler has been criticized as being too tough.

“Of all the regulators involved in the financial reform, he’s the one the financial industry is most concerned about and the one I most often hear the phrase, ‘He may go too far,’” Frank said.

Chilton, a Democrat, said Gensler’s inside knowledge of Wall Street has been invaluable, and he said that the CFTC would be worse off if Gensler recused himself from the MF Global matter.

“I think we would miss his expertise and counsel,” Chilton said.