NEW YORK (CNN/Money) - USA Interactive is making a $4.5 billion unsolicited bid for the remaining portions of three of its publicly traded units that it does not already own in a move to accelerate its expansion into e-commerce.

The units involved are online travel agent Expedia Inc. (EXPE: Research, Estimates); Ticketmaster (TMCS: Research, Estimates), which sells tickets to concerts, games and other live events; and Hotels.com (ROOM: Research, Estimates), which sells hotel rooms on the Web.

The USA Interactive proposal to increase its equity ownership to 100 percent in each of the subsidiaries was made to the units' top management at the weekend.

Stockholders of the three units may exchange their shares for USA Interactive stock "on the basis of conversion ratios that, in each case, reflect a premium of 7.5 percent to market as of the close of market on Friday, May 31," the company said.

"There will be lots of talk about whether the premiums we have offered are fair. We, of course, think they are, given the stock prices of our subsidiaries and USA in relation to each other," USA Chairman and CEO Barry Diller said. "But, more important, we believe a focus on the percentage premium we are offering is beside the point. This is an offer of stock, not cash. The real premium comes in the opportunity for the shareholders of our public subsidiaries to become owners of an even stronger USA."

Diller said he does not consider the bid for the shares "hostile," but that he looks forward to discussing the proposal with the subsidiaries' boards.

According to separate letters from Diller addressed to the boards of directors at each subsidiary, stockholders could exchange their shares for a set amount of USA Interactive shares. Expedia public shareholders, who own 46 percent of the company, would be offered the opportunity to exchange each share for about 2.7 USA shares. About 34 percent of Hotels.com is owned by shareholders, who could exchange their shares for about 1.81 of USA's, while Ticketmaster public shareholders, who also own about 34 percent of the ticket broker, could transfer each share for about 0.81 share of USA.

USA Interactive would have to issue about 156 million new shares to complete the transaction. Based on closing prices Friday, May 31, the exchange offer values each outstanding share of Expedia at $76.86, of Hotels.com at $51.48, and of Ticketmaster at $22.99.

"We've received the letter stating USA's intention to make an offer and are forming a special committee of independent directors to evaluate that offer on behalf of our shareholders," Expedia President and CEO Richard Barton said.

USA Interactive's announcement comes just three days after the company offered to buy Miami-based Interval International, a travel services company, for about $578 million.

Online travel companies recently went on acquisition sprees to boost market share, trying to cover all travel needs from airplane tickets and car rentals to tickets for sporting events and the theater.

With the Interval acquisition, USA Interactive reportedly will be better positioned in the time-share market to compete against leader Cendant Corp. (CD: Research, Estimates)

USA Interactive was created when USA Networks sold its cable TV properties and other entertainment units to Paris-based Vivendi Universal (V: Research, Estimates) a few weeks ago.

Shares of USA Interactive (USAI: Research, Estimates) gained 30 cents to close at $28.50 Friday.  

--from staff and wire reports