MidAmerican Energy Holdings Co. has reached a deal to acquire Baltimore-based Constellation Energy Group Inc. for approximately $4.7 billion, or $26.50 per share.

Constellation is a Fortune 125 company with 2007 revenues of $21 billion. It is the nation’s largest competitive supplier of electricity to large commercial and industrial customers and the nation’s largest wholesale power seller.

The deal, approved by both companies’ boards of directors but still subject to shareholder and regulatory approval, is expected to close in nine months. Upon signing a definitive merger agreement, which is expected by the close of business tomorrow, Constellation Energy will issue $1 billion of preferred equity yielding 8 percent to MidAmerican.

MidAmerican plans to allow Constellation Energy to operate autonomously and continue its current strategic plan.

Constellation’s stock has fallen from around $100 at the beginning of the year. It hovered around $50 until this week, when it dipped to $13.