Gannett, the publisher of USA Today, Des Moines Register and Iowa City Press-Citizen, has raised its unsolicited offer for Tribune Publishing Co to $15 per share from $12.25, valuing the publisher of the Chicago Tribune and the Los Angeles Times at about $475 million.Including debt, the offer is worth about $864 million.Tribune’s board adopted a shareholder rights plan earlier this month to thwart Gannett.Gannett said the latest bid followed an analysis of Tribune’s debt and pension liabilities.“In addition, after further review, Gannett has greater confidence in its ability to yield additional operational improvements in this transaction,” the company said.Tribune said it would thoroughly review the revised offer.Oaktree Capital Group LLC, the third-largest shareholder in Tribune, urged the company to negotiate a deal with Gannett earlier this month.