Upstream activator sequence. A DNA sequence some distance in front of a gene that increases transcription of the gene. Such sequences often stimulate binding of RNA polymerase to the promoter.
User Agent Server. Logical part of SIP implementation which receives SIP requests and generates SIP responses. See also: downstream, UAC, upstream
User Agent Server. In SIP, a server application that contacts the user when a SIP request is received, then returns a response on behalf of the user. The response accepts, rejects or redirects the request.