The Union Trust Building, a Downtown landmark that served for 20 years as an operations center for Mellon Bank, soon will become home to Siemens Engineering’s environmental engineering operations.

Siemens signed a 10-year lease last week with Los Angeles-based Mika Realty Group to occupy the third through the sixth floors, totaling 185,000 square feet. That is nearly one-third of the 11-story building’s 600,000 square feet of office and retail space. The deal was announced yesterday.

Mellon left the building in 2006. When Mika bought it in February for $24.1 million, it was 99 percent vacant, according to leasing broker CB Richard Ellis.

Mika also has engaged CBRE to carry out renovations to make the building qualify for LEED (Leadership in Energy and Environmental Design) certification by the U.S. Green Building Council. Renovations are scheduled to be completed by the end of the year.

Besides making the building more environmentally friendly and energy efficient, the renovations also will preserve its historical character, said CB Richard Ellis Vice President Jeremy Kronman.

Siemens employees will relocate from Downtown’s Oxford Centre and from the Crane Building in the Strip District to the Union Trust Building on Grant Street beginning later this year.

The relative speed with which Mika won the first tenants for the building is a good indicator for the local economy, Mr. Kronman said.

“Owning the building less than a year and already re-tenanting it is a very good testament for the region,” he said.