“…F-prime factors carrying S. typhimurium genes have not yet been isolated for all regions of the map of S. typhimuriun, so S. typhinuriun strains carrying F-prime factors containing E. coli genes (Table 3) have been used in a variety of studies, including: (i) complementation tests for allelism and, consequently, hints concerning linkage map position; (ii) dominance studies (190,497,657); (iii) diagnosis of enzyme quaternary structure (374); (iv) gene dosage studies (142, 657); (v) introduction of suppressors with known modes of action (406,430); (vi) description of Salmonella mutation type or of suppressor gene type (70, 188, 512; F. Winston and D. Botstein, personal communication); (vii) construction of P22 specialized transducing phages (257, 319); and (viii) detection of map locations of E. coli genes via gene dosage (422a) or electrophoretic mobility (234a) of the gene product in Salmonella. Low-efficiency transfer of some E. coli F-prime factors into S. typhimurium is due mainly to several restriction (hsd) barriers (26,107,108,137,237,455,569,632). The restriction can be lessened through admixture of a third, intermtediate, S. typhimurium host defective in restriction (e.g., strain SB5404 hisI4O4 hsd proC51 rpsL667 plated on media lacking histidine and proline [B.…”