The long-rumored deal brings YouTube‘s largest remaining independent content provider under the control of AT&T’s video joint venture with Peter Chernin. Fullscreen is a YouTube multi-channel network, or MCN, that provides YouTube creators with the technology and infrastructure they need to find advertisers and an audience. The company also has a significant talent management unit headed by former CAA executive Larry Shapiro and focused on taking YouTube creators into new business opportunities offline.

Fullscreen now claims a network of 50,000 content creators who reach 450M subscribers and account for 4B monthly views. The company has helped to promote YouTube stars the Fine Bros., Connor Franta, O2L, Andrea Russett, Lohanthony, Devin Supertramp and Jack and Jack.

It also has announced a string of recent signings of prominent creators such as JennXPenn to its management unit. Earlier this month, Fullscreen held its first live event, InTour in Pasadena, to spotlight creators and give them a chance to either perform or otherwise interact in person with their big fan bases.

The deal is the latest indication that mainstream media companies no longer consider YouTube networks to be a sideshow but an important investment opportunity, said Peter Csathy, CEO of consulting firm Manatt Digital Media.

This year, Disney bought Maker Studios for a price that could approach $1B while Warner Bros led an $18M investment round in in Machinima. In a slightly different category of online video company,  gamer-oriented live-streaming site Twitch sold to Amazon for $970 million. Last year, DreamWorks Animation bought AwesomenessTV, while Chernin acquired a controlling stake in anime provider Crunchyroll.

“Just looking holistically at the MCN world, it’s land-grab time,” Csathy said. “That’s what it is. The two big horizontal MCNs (Maker and Fullscreen) are now off the table.”

Next up, Csathy said, will be a push by big media companies and distributors to acquire at least a stake in major online networks with a narrower focus than the Makers and Fullscreens of the online video universe. Likely targets include fashion power Style Haul, sports site Whistle Sports, and Spanish-language-focused MiTu, all powerhouses with a more “vertical” approach to their programming.

“Those with positions for critical mass behind them, they’re in a great position,” Csathy said. “They’re focused. There will be a deal there” because media companies will have “a fear of missing out.”

Chernin has been an adviser and investor since Fullscreen was founded in 2011 by former YouTube exec George Strompolos, who will remain CEO and hold “a material ownership stake” in the company. Another investor, ad company WPP, also will remain a “strategic shareholder.” Today’s announcement doesn’t disclose financial terms of the deal, but Bloomberg says the deal will value Fullscreen at as much as $300M.

The deal matters, Strompolos says in a blog post, because Chernin “brings unparalleled entertainment experience to Fullscreen, which is critical as our world continues to merge with the broader entertainment industry. AT&T is a worldwide leader in mobile, which is a key area of interest for us, and their TV footprint is poised to expand significantly [with the planned acquisition of DirecTV]. WPP will continue to serve as our valued partner in the world of advertising and marketing.”

Strompolos adds that Fullscreen hopes to evolve into “a major player in the future of ‘over-the-top’ [industry jargon for Internet-delivered] video” that targets young adults.

Chernin echoes that view, saying that Fullscreen’s “support for content creators, its relationship with YouTube and its passion for delivering entertainment to youth audiences around the world are extremely appealing to us.”