Orientation of the histidine operon in the Salmonella typhimurium linkage map.

Abstract

A short segment of the Salmonella chromosome, the histidine operon,l 2 is considered to function as a unit structure in transcription from DNA to messenger RNA.3 4 Organized translation of this multigenic messenger molecule is then presumed to result in the formation of each of the proteins utilized in the ten reactions specific to the pathway of L-histidine biosynthesis in Salmonella. Some features of genetic fine structure of the histidine operon have been elucidated by means of transduction tests.l' 2 The histidine operon is a polarized structure with one the operator (hisO) end, arbitrarily considered as the beginning end in this report.tl 4 The location of the histidine operon in the bacterial linkage map has been determined from results of bacterial crosses performed utilizing either colicine-factor-mediated conjugation' or conjugation between Hfr donors and suitable recipient bacteria.' While the histidine operon appears to be a linear segment of a much larger linear structure, the bacterial chromosome, neither published experiments involving the genetic fine structure nor those concerned with the gross chromosomal positioning of the histidine operon has given indication of the orientation of the histidine operon with relation to other genes on the chromosome. Knowledge of the orientation is desirable since, for example, each operon may not function completely independently of certain neighboring operons. Furthermore, one questions whether a single one of the two DNA strands is transcribed for a considerable portion or the totality of its length, or whether transcription proceeds in a more complicated fashion.7 The present report describes results of colicine-factor-mediated genetic crosses which indicate that the histidine operon is oriented in a clockwise direction in the chromosome of Salmonella as it is customarily drawn.6 Dr. Kenneth E. Sanderson independently has arrived at a similar conclusion, based on F-factor-mediated conjugation tests.' Materials and Methods.-Table 1 lists the strain numbers and genotypes of the derivatives of Salmonella typhimurium, strain LT-2, used in this study. Wild-type LT-2 is streptomycinand azide-sensitive and possesses somatic antigens 4, 5, 12 and flagellar antigens i; 1, 2. The wild type has no requirements for the exogenous growth factors shown in Table 1 and can ferment each of the sugars listed. Strains with SL stock numbers were generously provided by Dr. B. A. D. Stocker, Lister Institute for Preventive Medicine, London. Three of these strains contain genes for flagellar antigens that have been transduced into Salmonella typhimurium from other Salmonella species.813 Strain SB93 is derived from strain SL802; the metG319 mutation was transduced into SL802 with P22 phage, and a methionine-requiring transductional clone, genetically identical to metG319 in transduction tests, was isolated following penicillin selection.