Differential Effect of Tetrazolium Dyes upon Bacteriophage Plaque Assay Titers

This study examined whether the practice of incorporating either tetrazolium red or tetrazolium violet dye into plaque assay medium deleteriously influences plaque assay titers. Representative members of six different virus families were studied: Cystoviridae (ϕ6), Leviviridae (MS2), Microviridae (ϕX174), Myoviridae (T2), Podoviridae (P22), and Siphoviridae (Denver, T1, and VD13). Each of the members of the Podoviridae and Siphoviridae families appeared to be suppressed by either one or both dyes at a 300-μg/ml concentration. The chosen representatives of the other bacteriophage families were not suppressed by either dye at a 300-μg/ml concentration. Subsequent trials revealed no suppression of Podoviridae or Siphoviridae plaque assay titers when members of these virus families were tested with the same two dyes at the lower concentrations of 150 and 50 μg/ml. Interestingly, the bacteriophage families whose members were affected by the dyes have additional commonality in that they are the two bacteriophage families whose members possess both double-stranded DNA genomes and noncontractile tails.     

KAT1 inactivates at sub-millimolar concentrations of external potassium

Structural analysis of K+ channel pores suggests that the selectivity filter of the pore is an inherent sensor for extracellular K+ (Ko+); channels seem to be inactivated at low Ko+ because of a destabilization of the conducting state and a collapse of the pore. In the present study, the effect of depleting Ko+ on the activity of a plant K+ channel, KAT1, from Arabidopsis thaliana was investigated. This channel is thought to be insensitive to Ko+. The channel was therefore expressed in mammalian HEK293 cells and measured with patch clamp technology in the whole cell configuration. The effect of Ko+ depletion on channel activity was monitored from the tail currents before, during, and after washing Ko+ from the medium. The data show that a depletion of Ko+ results in a decrease in channel conductance, irrespective of whether K+ is simply removed or replaced by either Na+ or Li+. Quantitative analysis suggests that the channel has two binding sites for K+ with the dissociation constant in the order of 20 microM. This high sensitivity of the channel to Ko+ could serve as a safety mechanism, which inactivates the channel at low Ko+ and, in this way, prevents leakage of K+ from the cells via this type of channel.