The phototropic bacterium sp. strain PCC 6803 is able to adapt its morphology in order to survive in a wide range of harsh environments. Under conditions of high salinity, planktonic cells formed cell aggregates in culture. Further observations using crystal violet staining, confocal laser scanning microscopy, and field emission-scanning electron microscopy confirmed that these aggregates were biofilms. Polyamines have been implicated in playing a role in biofilm formation, and during salt stress the content of spermidine, the major polyamine in , was reduced. Two putative arginine decarboxylases, Adc1 and Adc2, in were heterologously expressed in and purified. Adc2 had high arginine decarboxylase activity, whereas Adc1 was much less active. Disruption of the genes in resulted in decreased spermidine content and formation of biofilms even under nonstress conditions. Based on the characterization of the mutants, Adc2 was the major arginine decarboxylase whose activity led to inhibition of biofilm formation, and Adc1 contributed only minimally to the process of polyamine synthesis. Taken together, in the shift from planktonic lifestyle to biofilm formation was correlated with a decrease in intracellular polyamine content, which is the inverse relationship of what was previously reported in heterotroph bacteria. There are many reports concerning biofilm formation in heterotrophic bacteria. In contrast, studies on biofilm formation in cyanobacteria are scarce. Here, we report on the induction of biofilm formation by salt stress in the model phototrophic bacterium sp. strain PCC 6803. Two arginine decarboxylases (Adc1 and Adc2) possess function in the polyamine synthesis pathway. Inactivation of the and genes leads to biofilm formation even in the absence of salt. The shift from planktonic culture to biofilm formation is regulated by a decrease in spermidine content in This negative correlation between biofilm formation and polyamine content, which is the opposite of the relationship reported in other bacteria, is important not only in autotrophic but also in heterotrophic bacteria.

Arabidopsis thaliana contains five tandem-pore domain potassium channels, TPK1-TPK5 and the related one-pore domain potassium channel, KCO3. Although KCO3 is unlikely to be an active channel, it still has a physiological role in plant cells. TPK2 is most similar to KCO3 and both are localized to the tonoplast. However, their function remains poorly understood. Here, taking advantage of the similarities between TPK2 and KCO3, we evaluated Ca 2+ binding to the EF hands in TPK2, and the elements of KCO3 required for K + channel activity. Presence of both EFhand motifs in TPK2 resulted in Ca 2+ binding, but EF1 or EF2 alone failed to interact with Ca 2+ . The EF hands were not required for K + transport activity. EF1 contains two cysteines separated by two amino acids. Replacement of both cysteines with serines in TPK2 increased Ca 2+ binding. We generated a two-pore domain chimeric K + channel by replacing the missing pore region in KCO3 with a pore domain of TPK2. Alternatively, we generated two versions of simple one-pore domain K + channels by removal of an extra region from KCO3. The chimera and one of the simple onepore variants were functional channels. This strongly suggests that KCO3 is not a pseudogene and KCO3 retains components required for the formation of a functional K + channel and oligomerization. Our results contribute to our understanding of the structural properties required for K + channel activity.