Abstract
Evidence is presented for two natural transcription initiators positioned internally in the histidine operon of Salmonella typhimurium. They were detected using highly polar S. typhimurium his mutants as recipients for F' Escherichia coli his episomes. These intergeneric complementation tests provide a sensitive method for the detection of other initiators and of terminators. That at least part of the operator for the his operon lies distal to the transcription initiator is suggested by transduction analysis of a new highly polar mutant described in this paper.