Alexandria Real Estate Equities Inc. agreed to pay $138.5 million to Seattle for the roughly three-acre Mercer Mega Block near Amazon.com Inc.'s campus in what is expected to be the city's largest-ever real estate transaction, Bloomberg News reported, citing Mayor Jenny Durkan.

The office real estate investment trust is planning a $1 billion project at the site, with two new office buildings, a community center and an apartment tower containing at least 175 affordable units, the news outlet added.