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When Glu-325 in the lac permease of Escherichia coli is replaced with Ala, lactose/H+ symport is abolished. Thus, the altered permease catalyzes neither uphill lactose accumulation nor efflux. Remarkably, however, permease with Ala-325 catalyzes exchange and counterflow at completely normal rates. Taken together with the results presented in the accompanying paper [Püttner, I. B., Sarkar, H. K., Poonian, M. S., & Kaback, H. R. (1986) Biochemistry (preceding paper in this issue)], the findings suggest that the His-322 and Glu-325 may be components of a charge-relay system that plays an important role in the coupled translocation of lactose and H+.

The lac permease of Escherichia coli was modified by site-directed mutagenesis such that Arg-302 in putative helix IX was replaced with Leu. In addition, Ser-300 (helix IX) was replaced with Ala, and Lys-319 in putative helix X was replaced with Leu. Permease with Leu at position 302 manifests properties that are similar to those of permease with Arg in place of His-322 [Püttner, I. B., Sarkar, H. K., Poonian, M. S., & Kaback, H. R. (1986) Biochemistry 25, 4483]. Thus, permease with Leu-302 is markedly defective in active lactose transport, efflux, exchange, and counterflow but catalyzes downhill influx of lactose at high substrate concentrations without H+ translocation. In contrast, permease molecules with Ala at position 300 or Leu at position 319 catalyze lactose/H+ symport in a manner indistinguishable from that of wild-type permease. By molecular modeling, Arg-302 may be positioned in helix IX so that it faces the postulated His-322/Glu-325 ion pair in helix X. In this manner, the guanidino group in Arg-302 may interact with the imidazole of His-322 and thereby play a role in the H+ relay suggested to be involved in lactose/H+ symport [Carrasco, N., Antes, L. M., Poonian, M. S., & Kaback, H. R. (1986) Biochemistry 25, 4486].

Students encountered a wide variety of ethical and professional issues that can be used to guide pre-clinical and clinical education. Comparison of our findings with results from similar studies suggests that the wording of an assignment (specifying "ethical" issues, "professional" issues, or both) may influence the kinds of issues students identify in their experience-based clinical narratives.

The outpatient behavioral intervention demonstrated promising outcomes in a chronically mentally ill patient whose polydipsia had underlying psychogenic and physiological components. This case highlights the efficacy of combining behavioral and medical interventions.