The recent application of electrophoretic techniques to the separation of ribosomal proteins has provided a means by which differences in the protein composition of ribosomes may be readily detected. '-4 In a survey of the ribosomal proteins from various bacterial strains by one such technique, electrophoresis on polyacrylamide gel columns, it was observed that of all the strains of E. coli examined, K12 strains were distinctly different from the rest. To simplify the discussion we refer to this difference in K strains as "K-ribosomal character" or "K-character." It was obvious that such electrophoretic differences might prove to be useful for genetic studies involving the chromosomal localization of a ribosomal character.The present paper describes first the nature of the "K-character" and its prevalence among various strains of E. coli. Secondly, the electrophoretic difference has been used for the assignment of a genetic locus for a ribosomal protein on the bacterial chromosome.Materials and Methods.-Bacterial strains and growth media: K12 strains: HfrC, W1485, W1485(X), KB (Benzer), C600 (Appleyard), JC12A (Hfr), AB313 (Hfr), AB331, CR63, P678, K12 (Tatum), 58, Y-9, 679-680; B strains: B, B/r, B Berkeley; C strains: C20, C129 (Hfr), C409; other strains: 15, W, ML35. In addition, a K12 strain, AB312PL, was prepared for this study since a strain with the Hfr characteristics of AB312 but strs was desired. It was derived from a 2-hr uninterrupted mating between AB312 (Hfr) and AB470 (F-)j and has the following characters: Hfr: thi-, strs, xyl-, mt-, gal-, lac-, pro-, Xa-. The origin and direction of insertion of its chromosome are indicated by the inner circle in Figure 3.Bacteria, either for the preparation of ribosomes or for genetic crosses, were grown in L broth. Minimal media employed contained M9 salts solution,6 0.2% glucose, 5 jug/ml thiamine, 1.5% Bacto-agar, and when appropriate, amino acids at 20 ,g/ml. EMB agar: 2.74% Levine eosinmethylene blue agar without lactose (Baltimore Biological Lab.) supplemented with 1.0% of the appropriate sugar.Bacterial crosses: Hfr and F-strains were grown at 370 in L broth to a density of about 2 X 108 cells/ml. Hfr, 0.8 ml, and 4.2 ml of the F-culture were mixed in a blank Petri dish and incubated at 370 without agitation. After 1 hr, samples of the mixture were centrifuged, resuspended in saline, and suitable dilutions were plated on minimal agar supplemented with the appropriate amino acids. Colonies appearing after 48 hr were picked and restreaked on the same media to purify them. Single colonies were then checked for nonselected nutritional markers, sensitivity to dihydrostreptomycin (100 gg/ml), and fermentation characteristics. The latter were checked by streaking on EMB-agar containing either maltose, xylose, or mannitol. Transduction with phage Plkc: Lysates of bacteriophage Plkc, prepared by growth on a strRdonor bacterium using the plate lysis method of Arber,7 were added to a strs-recipient at a virus/ bacteria input of 2 in the presence of 2.5 X 10-3 M CaC...