[ by Melanie Gretchen ] MF Global's end may be in sight, and it's not a pretty view.  To date, assistant treasurer Edith O'Brien has asserted her Fifth Amendment privilege to avoid criminal prosecution.  What's her message?  That she has the ability to work out a deal with prosecutors.  What's her price?  A deal, of course. The sound of silence... may be broken, if what light Ms. O'Brien has to shed on the firm’s final days warrants the possibility of immunity in exchange for cooperation or a plea bargain for a reduced sentence.  Before any deal can come into play, prosecutors need to know what the witness will say and how credible that testimony is. Getting the Goods. Toward making a deal, prosecutors and defense lawyers participate in a a "proffer" session, where in the witness is interviewed and the information being offered is gauged for its usefulness.  Under the law, the Fifth Amendment privilege is lost after someone talks with the government, which could breakdown any deal.  To create opportunity for each side, the Justice Department will offer an informal letter immunity agreement (sometimes called "Queen for a Day" immunity after the 1960s television game show) - whereby the witness provides complete information to the government without waiving the protections of the Fifth Amendment. The Queen Indeed. Although Ms. O'Brien has not made a proffer to the government, she may indeed hold all the cards.  As Peter J. Henning of the New York Times put it: "She may be the one person who can explain how customer money was transferred out of MF Global, and what directions were given by the firm’s senior management on the issue.  That gives her significant leverage to work out an agreement that will not involve any criminal charges, either through a grant of immunity or informal assurances that she will not be prosecuted." Does the government have enough to threaten her with prosecution?  Does she have enough for the government to grant her immunity so she will make a proffer?  The ball's in the Justice Department’s court. For further details, go to [Dealbook, 3/29/12].