One of the nation’s biggest concert and sports promoters said it is joining forces with a start-up technology company to sell tickets to events at 105 arenas and theaters, in what could become a broader battle with Live Nation Entertainment Inc.’s Ticketmaster.

The promoter, Anschutz Entertainment Group, had been expected to distance itself from Ticketmaster since early last year, when the Justice Department approved the ticketing giant’s merger with Live Nation Inc., AEG’s only larger rival in concert promotion.

In approving the merger, antitrust regulators required that AEG be allowed to compete with Ticketmaster for ticket sales. As part of that requirement, Ticketmaster permitted AEG, a subsidiary of Anschutz Co., to license its ticketing software. Instead of licensing Ticketmaster’s system, however, AEG this week entered a joint venture called Outbox Enterprises.

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