In S. typhimurium the sites of mutations causing resistance to endproduct inhibition by tryptophan of the activities of anthranilate synthetase and phosphoribosyl transferase will be mapped in the first and second genes of the tryptophan operon. The degree of resistance and position of each mutation will be correlated with the position and the nature of the resultant change in amino acid sequence of the mutant enzyme and the degree of any modification in the tryptophan-binding affinity. The role of supX mutations in suppressing promoter mutations and altering the expression of non-mutant genes will be analyzed to identify the supX gene product, the nature of its activity and the genetic regulatory components with which it interacts, using both in vitro (e.g. cell-free coupled transcription-translation) and in vivo (e.g. maxi-cell) systems. The formation, structure and excision of chromosomal tandem duplications will be analyzed by genetic mapping techniques and the use of mutations to identify the processes involved.