Cultures of Escherichia coli K-12 malA-Xr galcan be transduced to gal+ by bacteriophage Xdg because of leakiness of the Xr phenotype. The efficiency of such transduction is about 10-5 that of transduction of mal+XB bacteria. Leaky cells (X' phenocopies) adsorb only very few phage particles, and many transductants, therefore, are defective heterogenotes or show integration of the gal+ gene, which is unaccompanied by lysogenization. Mutations affecting maltose metabolism in Escherichia coli K-12

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... re often pleiotropic, conferring on the bacteria resistance to phage X as well (9). Such mal-mutations occur in the malA and in the malB genes (10, 11). Mutants showing resistance to phage X are not necessarily impaired in their maltose-utilizing mechanism. One such mutant is strain CR63 of E. coli K-12 (2). Schwartz (11) was able to select Xr mutations which could be mapped in the malB region but which had no effect on maltose metabolism. Another class of X-resistant mutants, the /3,X one, was described by Curtiss (7) and found by him to be connected with chromosomal aberrations in various regions of the genome. We have isolated a number of mal-Xr mutants in E. coli K-12 and have screened them for reversion to X sensitivity by using X-mediated transduction as a selective tool. As we report here, all mal-Xr mutants studied by us so far were found to be transducible by phage X with low, but fixed, efficiencies. This transduction does not result from and is more frequent than back mutation to X sensitivity. The present report describes some peculiar properties of the Xr bacteria that are phenotypically sensitive to the phage. MATERIALS AND METHODS Media. Cultures were grown in nutrient broth [NB: 8 g of Nutrient Broth (Difco), 5 g of NaCl, 1 liter of water] or in M9B medium (3 g of KH2PO4, 7 g of Na2HPO4, 1 g of NH4Cl, 2 mm MgSO4, 0.1 mM CaCl2, 4 g of glucose, 1 liter of water) supplemented with Casamino Acids (Difco; 10 g/liter).