Two of Madison Avenue’s most familiar media-buying units — Starcom MediaVest Group and ZenithOptimedia Group — are no longer, restructured as part of French ad giant Publicis Groupe’s strategy to make the company’s vast media-agency holdings more nimble.

Publicis said it would break up the two global holdings and reorganize other assets in favor of operating four outlets: Starcom; Zenith; Mediavest Spark; and Optimedia Blue 449. All four will serve ad clients around the world.

The move is expected to break down walls that had built up between Starcom, Spark and Mediavest, which had been housed under Starcom Mediavest; and Zenith and Optimedia, part of the separate ZenithOptimedia. The new structure is aimed at lending an appearance of heft, since the power of the money all the agencies wield will be collected under one umbrella, rather than being broken up between two.

The restructuring comes as U.S. TV media companies prepare for the annual upfront market, when they try to sell the bulk of their ad inventory for the coming season. Agencies like the ones owned by Publicis are instrumental in negotiations for placement of billions of dollars across CBS, NBCUniversal, 21st Century Fox, Walt Disney and other media titans. Publicis did not offer specifics on whether its maneuver would require changes in how various accounts are handled. Among the advertisers the company serves are Samsung Electronics and Yum Brands’ Taco Bell.

The effort takes place after one of the agencies, Mediavest, lost significant amounts of business from longtime clients Procter & Gamble, Coca-Cola and Walmart.

The agencies, collected in a unit known as Publicis Media, will represent $39 billion worth of ad spending, representing a third of the U.S. market, the company said.

Each agency will be led by a global brand president. Lisa Donohue, previously CEO of the Starcom unit, will serve as its global brand president. Vittorio Bonori will lead Zenith. Brian Terkelsen will lead Mediavest Spark and Andras Vigh will lead Optimedia Blue 449. But each agency will also have a CEO overseeing U.S. operations: Chris Boothe will become CEO of Mediavest | Spark; Dave Ehlers will lead  Optimedia  Blue 449; Lou Rossi will continue to lead Zenith; and Donohue will continue to lead Starcom until a successor is named. All U.S. brand leaders will report to Tim Jones, CEO of Americas.

Dave Penski will become chief investment officer for Publicis Media in the U.S., overseeing all media investment and media vendor partnerships, and will also report to Jones.

The restructuring represents the first big effort by Steve King, who was recently appointed to lead the Publicis media assets.