POTASSIUM FLUXES IN CHLAMYDOMONAS-REINHARDTII .2. COMPARTMENTAL ANALYSIS

K-42(+) and Rb-86(+) were used to determine the subcellular distributi on of potassium in Chlamydomonas reinhardtii by compartmental analysis . In both wild type and a mutant strain, three distinct compartments ( referred to as I, II, and III) were apparent. Using K-42(+), we found that these had half-lives for K+ exchange of 1.07 min, 12.8 min, and 2 .9 h, respectively, in wild-type cells and 0.93 min, 14.7 min, and 9.8 h, respectively, for the mutants. Half-lives were not significantly d ifferent when Rb-86(+) was used to trace K+ Compartments I and II prob ably correspond to the cell wall and cytoplasm, respectively. Based on the lack of a large central vacuole in Chlamydomonas, the effect of a dark pretreatment on the kinetic properties of compartment III and th e similarity between the [KC] of compartment III and that of isolated chloroplasts, this slowly exchanging compartment was identified as the chloroplast. Growth of wild-type cells at 100 mu M (instead of 10 mM K+) caused no change of cytoplasmic [K+] but reduced chloroplast [K+] very substantially. The mutants failed to grow at 100 mu M K+.