Spermidine Synthase is Required for Growth of Synechococcus sp. PCC 7942 Under Osmotic Stress Apiradee Pothipongsa 1 • Saowarath Jantaro 1 • Aran Incharoensakdi 1 Received: 16 June 2016 / Accepted: 18 July 2016 Ó Springer Science+Business Media New York 2016 Abstract The Synechococcus sp. PCC 7942 spermidine synthase encoded by spds gene (Synpcc7942_0628) is responsible for spermidine biosynthesis. Two Syne- chococcus strains, the overexpressing spds (OX-spds) and the spds knockout (Dspds), were constructed and charac- terized for their growth and photosynthetic efficiency under osmotic stress imposed by sorbitol. The growth of Dspds was completely inhibited when cells were grown in the presence of 400 mM sorbitol. Under the same condition, the OX-spds showed a slightly higher growth than the wild type. The OX-spds under osmotic stress also had a sig- nificant increase of spermidine level in conjunction with the up-regulation of the genes involved in spermidine biosynthesis. A higher ratio of spermidine to putrescine, an index for stress tolerance, under osmotic stress was found in the OX-spds strain than in the wild type. Overall results indicated that the spermidine synthase enzyme plays an essential role in the survival of Synechococcus sp. PCC 7942 under osmotic stress. Keywords Spermidine synthase Á Overexpressing spds Á spds knockout Á Synechococcus sp. PCC 7942 Á Osmotic stress Introduction Polyamines (PAs) are linear, flexible organic polycations involved in a wide range of biological processes, including growth, development, and programmed cell death [2, 5]. Putrescine (Put), spermidine (Spd), and spermine (Spm) are major PAs found in both prokaryotic and eukaryotic cells. Polyamines are highly responsive to various abiotic stres- ses such as salt and drought, both of which cause an osmotic stress, and fluctuating temperatures [1]. In general, the levels of Put decrease, while those of Spd and/or Spm increase under salinity condition [25]. However, the changes of free polyamine levels fluctuated among differ- ent kinds of species and stress duration [22]. Spd is more closely associated with the stress tolerance of plants than Put and Spm [5]. The compounds that have a high number of amino groups such as Spm and Spd are effective in the scavenging of reactive oxygen species (ROS) under stress conditions [21]. Polyamine biosynthesis occurs in several plants and bacteria where the Put is formed directly by ornithine decarboxylase (ODC; EC 4.1.1.17) or indirectly by argi- nine decarboxylase (ADC; EC 4.1.1.19) [13]. The sper- midine synthase (SPDS; EC 2.5.1.16) transforms Put and decarboxylated S-adenosylmethionine (dcSAM) into Spd. The dcSAM is generated from decarboxylation of S- adenosylmethionine (SAM) by SAM decarboxylase (SAMDC; EC 4.1.1.50). To clarify the role of polyamines on the adaptation to stress tolerance, the transgenic plants overexpressing SPDS have been shown to increase the tolerance to multiple stresses [9, 18]. The accumulation of polyamines, especially Spd, has been reported to be involved in the tolerance to salt and osmotic stress in cyanobacteria [15, 24]. However, the underlying mecha- nisms of stress tolerance concerning the involvement of & Aran Incharoensakdi aran.i@chula.ac.th 1 Laboratory of Cyanobacterial Biotechnology, Department of Biochemistry, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand 123 Curr Microbiol DOI 10.1007/s00284-016-1107-8