It is not known if Johnson will return to New Zealand to continue working for Ceres Environmental.
Montgomery Advertiser business and government editor Rick Harmon said parts of the corruption case involved claims Mississippi Indian tribes funnelled funds to anti-gambling forces in Alabama.
Johnson was of interest because he lambasted Riley during a 2010 campaign for Governor of Alabama. "When he ran for governor, he made statements saying that while he had been attached to the Riley administration, he had heard comments about contributions coming ... from Indian tribes," Harmon said.
Another source said Johnson's testimony would not be taken seriously because of fallout from the sperm donation story.
"Opposing counsel will destroy Johnson on the stand, thanks to your exposé. It will be easy to paint him as a joke - especially after Jay Leno painted him as a joke during his December 13 Tonight Show monologue."
One American political blogger alleged senior Republicans, possibly including Rove, tipped off the Herald on Sunday to Johnson's actions in Christchurch.