A small crowd, braving oppressive warm temperatures, watched Thursday as a white steel girder covered in signatures and topped with a small tree was put in place on top of the four-story Discovery One Square building.

The ceremonial “topping” was hosted by the building’s developer, Minneapolis-based M.A. Mortenson Co. The 89,000-square-foot One Discovery Square is under construction at the corner of Fourth Street Southwest and Second Avenue Southwest.

One Discovery Square is the first building to go up in Mayo Clinic’s 16-block Discovery Square subdistrict of the Destination Medical Center initiative.

One Discovery is anticipated to be the nucleus for a series of commercial science buildings within the subdistrict.

Mayo Clinic and the University of Minnesota Rochester are the two anchor tenants that have been announced as leasing space in the complex. Mayo will occupy 30,000 square feet and UMR will fill 9,383 square feet of the building.

Jeremy Jacobs, who is shepherding the Mortenson project, said during the event that a number of other tenants have signed letters of intent.

He estimated that two to four more tenants might be announced by the end of summer.

At this point, Jacobs said only about 30 percent of the building’s space remains uncommitted.