The induction of L-arabinose isomerase in Salmonella typhimurium (LT2) is repressed on infection with clear plaque forming mutants (C1 and C2) of the temperate phage P22 (C+). However, after infection with C+ leading to lysogeny, there is a temporary repression. During this period, messenger ribonucleic acid (RNA) for L-arabinose isomerase accumulates. DNA-RNA hybridization data suggest that there is transcription of host DNA during the period of repression. Interference at the level of

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... ion might be responsible for the cessation of induced enzyme synthesis. Salmonella typhimurium LT2 on infection with the temperate phage P22 (C+) produces phage deoxyribonucleic acid (DNA) before lysogeny is established (11, 12) . Phage-specific DNA synthesis begins at about 4 min after phage infection, continues to increase for 2 to 4 min, and then decreases until complete cessation occurs at 16 min. Phage-specific protein synthesis under conditions of lysogeny as well as lysis was also studied (3). In infections leading to lysis, phagespecific protein synthesis was detected from the 4th min after infection and continues throughout the latent period, whereas, in the case of infections producing lysogeny, the synthesis continues from 4th to 8th min, after which the rate decreases and becomes practically zero after the 20th min. Studies on the synthesis of an inducible enzyme, L-arabinose isomerase, in S. typhimuriwn have already been done (A. K. Bhattacharya and M. Chakravorty, unpublished data). To understand the differences in the regulation of host protein synthesis after infections leading to lysis or lysogeny, the induction of L-arabinose isomerase in phage-infected S. typhimurium has been studied. The results corroborate the suggestion (11, 12) that the first phase after infection leading to lysogeny resembles that of a lytic infection. Evidence will also be presented to indicate that the interference in enzyme synthesis during this phase is most probably at the stage of translation and not at transcription. Similar results have already been obtained for ,B-galactosidase synthesis (1, 5, 7, 10). MATERIALS AND METHODS Bacteria and bacteriophage strains. S. typhimuriwn strain LT2 and the temperate phage P22 (C+) were kindly provided by P. Margolin of the Cold Spring Harbor Laboratory of Quantitative Biology, Cold Spring Harbor, N.Y. The clear plaque mutants, C1 and C2, were generous gifts from M. Levine of the Department of Human Genetics, University of Michigan, Ann Arbor, Mich. The lysogenized strain of S. typhimuriwn carrying P22 as the prophage, LT2(C+), was isolated and purified in this laboratory after infecting the host with P22 at a high multiplicity of infection (MOI). Growth. The cells were normally grown at 35 C on a reciprocal shaker in minimal medium containing 0.2% glycerol. The composition of the minimal medium is as follows: K2HPO4, 10.5 g; KH2PO4, 4.5 g; (NH4)2S04, 1.0 g; sodium citrate-5H20, 0.47 g; MgSO4-7H20, 0.1 g; water, 1 liter. Growth was measured by following the optical density of the cell suspension at 640 nm in the Leitz colorimeter. Iduction of L-arabinose isomerase-Overnight cultures of LT2 were diluted 10 times in fresh medium and allowed to grow again. When the cultures had grown to an optical density of 0.6, they were again diluted to an optical density of 0.1. L-Arabinose (0.3%) was added as an inducer after the growing cell suspension reached an optical density of 0.3 (approximately 3 X 108 cells/ml). Samples were removed at intervals, and the enzyme induction was immediately stopped by adding chloramphenicol (50 ,ug/ml). Each sample was quickly cooled to 0 C, centrifuged, washed once with an equal volume of cold tris(hydroxymethyl)aminomethane (0.12 M; pH 8.0) and suspended in 1 ml of the same buffer. The cell suspensions were then treated with ethylenedia-541 on May 9, 2020 by guest