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Genes have been cloned from Salmonella typhimurium which when present on the multicopy plasmid pBR322 in the E. coli strain NT31 confer a Gua+ phenotype on this strain. NT31 is a purE gpt double mutant and it was expected that a Gua+ phenotype could be conferred on it by the cloning of either gpt or purE. It was, however found that in addition to these two loci the molecular cloning of another gene, which has been identified as hpt, in pBR322 confers a Gua+ phenotype on NT31. This result is explained by the overproduction of the hpt gene product, hypoxanthine phosphoribosyl transferase, which compensates for the lack of the gpt product guanine-xanthine phosphoribosyl transferase. Restriction analysis of the three loci, gpt, hpt and purE is also presented.

1. In the transductionpro-401 (×) + some of the transductants are surrounded by several hundred small wild-type satellite colonies; these transductants spontaneously release phage which transducespro-401 to wild-type at high frequency (HFT phage).2. When the HFT phage is used to infectpro-401 at very low multiplicities of infection, most of the transductants are defective lysogens and segregate proline-requiring phage-sensitive derivatives; these transductants are apparently heterogenotes. At higher multiplicities of infection, or with lysogenic recipients, a higher proportion of satellited transductants is found.3. The HFT phage preparations transduce only the proline region of the donor genome.4. The existence is inferred of a defective P22 particle specifically incorporating the proline region of theSalmonellachromosome; these defective particles can establish themselves as prophage and confer immunity upon the infected cell, but are unable to replicate unless a normal prophage is also present. Satellited transductants are lysogenic both for a normal and defective (proline region carrying) phage, and so on lysis release transducing phage.5. This system is compared with the λdg-galand P1-dl-lacsystems inE. coli.

TN experiments which were designed to measure the rate of mutation of a leucine auxotroph of Salmonella typhimurium to prototrophy it was found that the apparent" reversions " were of three types. These were distinguishable by their growth rates in the absence of leucine. This paper presents an analysis of one of these " reversions " which has a slower growth rate in the absence of leucine than the wild type. 2. TERMINOLOGY, MATERIALS AND METHODS The terminology and nomenclature used is that described by Demerec (1956). The symbol for each mutant is an abbreviation of the nutritional requirement involved try, requires tryptophane ; roe, requires methionine ; lee, requires leucine ; era, unable to utilise arabinose. Similar mutants of independent origin are distinguished by numbers, assigned to the mutants in the order in which they are found. When the locus of the mutant allele is known, a capital letter designating that locus is included in the symbol for the mutant; e.g. tryB-2. The symbol for a suppressor gene includes an abbreviation of the mutant character it suppresses together with an identifying number ; e.g. se-lees is the first isolated suppressor of leucine requirement. Symbols for wild type alleles are omitted except where ambiguity may arise, when the symbol for the mutant allele with a superscript plus sign is used. The triple auxotrophs leu-sr, meA-ec, tryB-2 and leu-s52, meA-22, tryB-2 were obtained from the double auxotroph meA-22, tryB-2 by irradiation with ultraviolet light followed by penicillin screening. The double mutant leu-39, ara-9 was obtained from the Cold Spring Harbor collection. All the stocks used were S. typhimurium strain LT-2. Temperate phage PLT-22 was used in all transduction experiments. Cultures were grown in i per cent. Difco nutrient broth and assayed on 255 per cent. Difco nutrient agar. The minimal media (MM) and enriched minimal media (EMM) were those of Yura (1956) with the addition of ooo2 per cent. of methionine and of tryptophane. Leucine supplemented media (MM+L and EMM+L) contained in addition ooo2 per cent. dl-leucine. For EMB agar, oo4 per cent. eosin, ooo6 per cent. methylene blue and I0 per cent. arabinose were added to 255 per cent. nutrient agar. T2 buffer contained KH5PO4 oi5 per cent., NaCI o4 per cent., K5504 o5 per cent., Na2HPO4 o3 per cent., Mg504 ooI2 per cent., CaCl5 ooos s per cent. and gelatin ooos per cent. 2.1. Preparation of phage The donor strain was grown for 24 hr. on a nutrient agar slope and the bacteria washed off in ml. of i per cent. nutrient broth. Equal volumes of this suspension and a phage suspension with a titre of approximately so particles/ml. were mixed and stood for 8 mm. to allow for adsorption of the phage, after which os ml. samples 6'

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