SUMMARY: The growth characteristics, accumulations and genetic tests carried out by means of complete transduction for over 200 independently isolated histidine-requiring () mutants of are described. Sites of mutation engendering the histidine-requiring phenotype all lie within a short chromosomal region. This region is divisible into seven smaller regions, or gene loci, each correlated with a specific physiological phenotype. The gene loci appear to be linearly arranged and are linked in the order and . The five central loci are linked in an order corresponding to the sequence of reactions in the biosynthetic pathway for histidine. The functions of the two terminal loci, and are unknown. The loci are nearly, or truly adjacent. A few of the sites of mutation are accurately mapped within a single gene. All the evidence is consistent with the view that processes involved in intra- and inter-genic recombination are identical. Some characteristics of multisite mutations are described. Several multisite mutations are interpreted as due to deletion of genetic material while one (-57) is interpreted as due to an inversion, with or without concomitant deletion of an adjacent region. Single-site and multisite mutations exert specific effects on recombination frequencies in nearby regions.


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