Effects of ph stress on polyamine content in cyanobacterium Synechocystis sp. PCC 6803

Abstract

Effect of pH stress on growth, intracellular pigments and polyamine contents in cyanobacterium Synechocystis sp. PCC 6803 was investigated. Long-term pH adapted condition in a range of pH 6.5 - 8.5 did not affect cell growth, chlorophyll a and carotenoid contents while pH 5.5 condition inhibited cell growth and the accumulation of intracellular pigments. For short-term stress of 4 h-period, acidic conditions at pH 2 and pH 4 influenced the significant decrease on cell growth compared to control. Moreover, acid stress at pH 2 highly inhibited the accumulation of chlorophyll a, as well as photosynthetic efficiency of Synechocystis cells. This result was corroborated by the decrease of chlorophyll a synthase gene transcript under acid stress treatment, thereby affecting on the changed color of cell culture. Free form-polyamine was found as a major form, and spermidine was present dominantly in all conditions. In addition, total free form-polyamines were maintained relatively under both tolerable acid and alkaline stresses whereas their bound form was mainly fluctuated. In particular, alkaline stress induced the titer of bound form-polyamines higher than acid stress did. The pH transition experiment was directed to the cellular adaptive mechanism against pH stress. Since the polyamine level of alkaline-stressed cells was immediately induced after transferring to pH 2 stress with unchanged ambient pH along treatment, the answer of first mechanism responded to acid stress was the internal pH balance of products from arginine catabolism, herein polyamines. In contrast, Synechocystis cells seemed to highly secrete polyamine in to culture medium corresponding to the increase of cell culture pH under alkaline condition. The amounts of both adc (including adc1 and adc2) gene, encoding arginine decarboxylase, and speB2 gene, encoding agmatinase, related to putrescine synthesis were determined. The transcript and protein levels were analyzed by RT-PCR and Western blotting, respectively. The level of biosynthetic genes including adc and speB2 mRNAs were reduced strongly under acid stressed condition whereas they were induced significantly under alkaline condition. However, ADC protein level was slightly decreased under acid stress. It was suggestedthat polyamine content induced by acid stress might be regulated at protein level. The potD gene relating to polyamine secretion showed a high reduction by acid stress whereas paogene, in polyamine degradation, was slightly increased after 120 min-treatment. For alkaline stress at pH 12, the polyamine biosynthesis genes of adc1 and adc2 mRNAs were induced obviously after 120 min-treatment whereas speB2 transcript amount was lower than that of control. A high induction of potD mRNA amount was observed significantly under alkaline stress whereas pao mRNA level was decreased comparing with that of control.