(Telecompaper) AT&T has agreed to sell the rights to 9,700 of its mobile towers to Crown Castle for USD 4.85 billion cash. Crown Castle will have the exclusive right to lease and operate the AT&T towers for an average 28 years. At the end of the lease, Crown Castle will have the option to buy the towers for USD 4.2 billion. Crown Castle said nearly half of the AT&T towers are located in the top 50 markets in the US, where the majority of network densification and upgrade activity is expected to occur. The towers average 1.7 existing tenants per site, allowing further room for growth. The deal increases Crown Castle's footprint by 33 percent, strengthening its place as the biggest tower operator in the US. The company expects the contract to be slightly accretive to 2014 adjusted funds from operations per share and add 5 percent over the long term. AT&T has contracted to maintain its communications facilities on the towers for a minimum of 10 years with monthly rent of USD 1,900 per site and fixed annual rent increases of 2 percent. AT&T will also have access to additional space on the towers for its future use, subject to certain restrictions. The transaction is expected to close in the fourth quarter.