Plant Science Letters, 22 (1981) 345--352 345 © Elsevier/North-Holland Scientific Publishers Ltd. Mn ~÷ AND Mg2÷ UPTAKE IN Mn-SENSITIVE AND Mn-RESISTANT YEAST STRAINS M.E. BIANCHI,M.L. CARBONE* and G. LUCCHINI lstituto di Genetica, Universitd di Milano, via Celoria 10, 20133 Milan (Italy) (Received February 16th, 1981) (Revision receivedMay 11th, 1981) (Accepted May 11th, 1981) SUMMARY Mn ~+ and Mg 2+ uptake was studied in a wild type strain of S. cerevisiae and in a mutant resistantto M n 2+. Contrary to expectations, the resistant strain accumulates Mn 2÷ to a higher concentration than the wild type strain, while Mg 2* uptake is unaffected. Moreover, the transmembrane electric potential (PD) of the resistant strain is far lessdepolarized by M n 2+ than that of the sensitivestrain.A possible explanation for these results isdis- cussed. INTRODUCTION In recent years great progress was achieved in the study of active transport systems in plants and microorganisms. It is now fairly well established that in fungi there exists an electrogenic pump that extrudes H+ at the expense of ATP hydrolysis: the electrochemical gradient generated in this way serves as primary energy source for active secondary transport systems [ 1]. The transport of many nutrilites, e.g. sugars in Rhodotorula gracilis [2,3], Neurospora crassa [4,5] and yeast [6,7], aminoacids in 8accharom~ces strains [6,8--10] and some anions, such as phosphate, in 8accharomyces cerev/s/ae [8], seem to be I-r4ymports. On the contrary, the transport of monovalent cations is supposed to be an electrogenic process, not directly coupled to proton import [11,12]. Very few studies have been concerned with the transport of bivalent *To whom corr~pondence should be stunt. Abbreviations: MIC, minimal inhibit?ry concentration; PD, tranmnembrane electric potmntial.