On Tuesday, rent-by-the-hour car company Zipcar (NASDAQ: ZIP) and the company planning to buy it, Avis Budget Group (NASDAQ:CAR), filed joint announcements with the SEC confirming that the period for Hart-Scott-Rodino Antitrust Act review of their pending merger by the Department of Justice expired on Monday.

Under the rules established by Hart-Scott-Rodino, companies subject to the act must file premerger notifications with the DOJ and then wait 30 days to see whether the DOJ requests further information. If a request is received, the DOJ gets additional time to review the additional materials. If the DOJ does not request further information -- as in this case -- this acts as approval by default, and the merger may proceed as planned.

Thus, Zipcar's merger with Avis has moved one step closer to completion. Approval by both companies' shareholders, and also antitrust clearance by U.K. authorities, remain to be cleared. But the DOJ has essentially given the transaction a green light.

Avis shares reacted positively to the news, rising 1.8% in Tuesday trading to close at $23.24. Zipcar shares gained a penny.