Virtu Financial Inc.’s bid to become one of the biggest companies in the high-frequency trading market got one step closer in late April with its proposed $1.4 billion all-cash acquisition of electronic trading rival KCG Holdings Inc.

The deal, announced on April 20 and expected to close in the third quarter, will result in New York-based Virtu controlling about 20 percent of the U.S. equity trading market. Paul, Weiss, Rifkind Wharton & Garrison is advising longtime client Virtu on the merger, which will unite two of the country’s largest electronic trading firms in a time of reduced market volatility. The transaction has also yielded roles for a half-dozen Am Law 100 firms.