Abstract
Since betaine is synthesized in chloroplast, its transport among plants would be important. However, the molecular mechanisms of betaine transport in plants are largely unknown. Here, we isolated betaine transporter from a halotolerant cyanobacterium Aphanothece halophytica (ApBetT) and its properties were compared with those of mangrove Avicenia marina (AmBetT). It was found that ApBetT specifically catalyzed the transport of betaine. Sodium markedly enhanced betaine uptake rates whereas potassium and other cations showed no effect, suggesting that ApBetT is a Na+-betaine symporter whereas AmBetT is a H+-betaine symporter. pH dependence of betaine uptake activities of ApBetT were high at alkaline pH whereas those of AmBetT were high at low pH. The fresh water cyanobacterium, Synechococcus cells overexpressing ApBetT showed NaCl-activated betaine uptake activities with enhanced salt tolerance allowing growth in sea water supplemented with betaine. Based on these data, physiological role of ApBetT and AmBetT will be discussed.
