Abstract
A RelA/SpoT homolog, HpRSH, was identified in Haematococcus pluvialis. HpRSH was found to catalyze Mg2+-dependent guanosine tetraphosphate (ppGpp) synthesis and Mn2+-dependent ppGpp hydrolysis, respectively. The transcription of HpRSH was significantly upregulated by environmental stresses, such as darkness, high light, nitrogen limitation, and salinity stress. The intracellular ppGpp level was also increased when exposed to these stresses. In addition, the classical initiator of stringent response, serine hydroxamate (SHX), was found to upregulate the transcription of HpRSH and increase the level of ppGpp. Moreover, stringent response induced by SHX or environmental stresses was proven to induce the accumulation of astaxanthin. These results indicated that stringent response regulatory system involved in the regulation of astaxanthin biosynthesis in H. pluvialis. Furthermore, stringent response was unable to induce astaxanthin accumulation under dark condition. This result implied that stringent response may regulate astaxanthin biosynthesis in a light-dependent manner.
Highlights
Haematococcus pluvialis is a unicellular biflagellate green microalga that inhabits in freshwater (Lorenz and Cysewski, 2000)
HpRSH was found to possess double functions and catalyzes Mg2+-dependent ppGpp synthesis and Mn2+dependent ppGpp hydrolysis. Both the transcription of HpRSH and the level of ppGpp were raised by serine hydroxamate (SHX) and environmental stresses, such as darkness, nitrogen starvation, high light, and salinity stress
Stringent response induced by SHX or environmental stresses led to a significant accumulation of astaxanthin
Summary
Haematococcus pluvialis is a unicellular biflagellate green microalga that inhabits in freshwater (Lorenz and Cysewski, 2000) This alga attracts considerable attention because it accumulates the highest content of astaxanthin (up to 4% by dry weight), which is a high-value carotenoid with excellent antioxidant activity (Han et al, 2013). Accompanied by the accumulation of astaxanthin, this alga exhibits a dimorphic life cycle, mobile green vegetative cells in favorable environment and non-mobile red cyst cells in unfavorable environment (Droop, 1954; Zhang et al, 2017). During this process, H. pluvialis stops dividing, enters into a resting stage, and maintains a lower metabolic rate (Boussiba and Vonshak, 1991). Many studies focus on the relationship between astaxanthin accumulation and environment stresses (Boussiba et al, 1999; Huir, 2000; Su et al, 2014), less is known about
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