NEW YORK ( TheStreet) -- Shares of Southern Union ( SUG) soared late Thursday after receiving an all-cash buyout proposal from Williams ( WMB) that values the natural gas pipeline company at $4.9 billion, or $39 a share.

William's bid tops an existing agreement Southern Union has to be acquired by Energy Transfer Equity ( ETE)(ETE_) for $4.2 billion, or $33 a share.

Both the Williams' and Energy Transfer offers include the assumption of $3.7 billion of debt.

In a statement late Thursday, Southern Union said its board will "review the Williams' bid " in due course, consistent with fiduciary duties and in accordance with the terms and conditions of Southern Union's previously announced merger agreement with Energy Transfer."

Shares of Southern Union rose to $39.20, up 14.8%, in after-hours trading on Thursday.

-- Written by Joseph Woelfel

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