Viacom and CBS Corp. are again joining forces to bolster their positions in a rapidly changing industry that's pushing other media companies to bulk up. The two first merged in 1999 and broke apart in 2005. Now, they are ViacomCBS, with a collection of cable channels, broadcast networks and film and publishing divisions.

Among the brand-name media assets Viacom brings are BET, Comedy Central, MTV and the movie studio Paramount. CBS properties include the CBS Television Network, CBS News and 15 CBS-owned TV stations, as well as the cable network Showtime and the publisher Simon & Schuster.

ViacomCBS Chairman Shari Redstone and ViacomCBS President and CEO Bob Bakish celebrated the new corporate combination Thursday morning with a ceremonial ringing of the opening bell at the Nasdaq Market Site in New York's Times Square. The company's Nasdaq ticker symbol: VIAC. Shares fell 1% in early trading to $40.56.

ViacomCBS, valued at about $30 billion when the merger was announced in August, is just the latest in a string of blockbuster media marriages. Walt Disney Co. bought most of 21st Century Fox in a deal valued at $52 billion. AT&T completed its acquisition of Time Warner for $85 billion. Comcast bought a majority of European pay-TV operator Sky News in for $40 billion.

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These established players are also competing against internet companies including Amazon and Netflix, each of which have at least 100 million paying subscribers and are pouring billions into creating movies, series and other content that's streamed on home televisions and mobile devices.

And the first generation of streaming services is itself facing more newcomers. Last month Apple and Disney each launched streaming services costing subscribers less than $10 a month. AT&T and NBCUniversal are also planning to launch streaming services next year that will compete with subscription offerings like CBS All Access.

Both CBS Corp. and Viacom have long been controlled by Sumner Redstone, 96, and more recently his daughter, Shari, 65, through their family's privately owned company, National Amusements, which operates movie theaters in the U.S., U.K. and Latin America. Shari Redstone is the president of National Amusements and served as vice-chairwoman of both CBS and Viacom before their merger. On Wednesday, she became chair of the combined ViacomCBS board, making her arguably the most powerful woman in the media industry.

Shari Redstone in July 2019. As the board chair and controlling shareholder of the newly combined ViacomCBS, she is arguably the most powerful woman in today's media industry. Reuters

The two companies had sought a merger several times in recent years, but their management teams' differences were never bridged. Then the CBS board went through a dramatic overhaul last year following the departure of longtime CEO Leslie Moonves over misconduct allegations. The changes paved the way for the merger made official Wednesday.

Bakish, chief executive of Viacom, becomes president and CEO of the combined ViacomCBS. His initial contract lasts four years, according to filings with the Securities and Exchange Commission.

"This is a historic moment that brings together two iconic companies to form one of the world's most important content producers and providers," Bakish said in a statement Wednesday.

Joe Ianniello, acting chief executive officer of CBS Corp., becomes chairman and CEO of CBS, overseeing all CBS-branded assets in his new role at ViacomCBS, the companies said.

The combination has its controversies. A CBS Corp. investor sued the companies last month seeking files on the deal negotiations and alleging the merger price "unfairly benefits" Shari Redstone as controlling shareholder.

The company could still be on the hook to pay $120 million to ousted CEO Moonves, who filed an arbitration case earlier this year claiming he was fired without cause. And CBS has been criticized for recently awarding a $100 million "severance" payment to Ianniello, despite the fact that he will remain at his job.