Sequence comparison and expression analysis of an inferred Na+/Pi cotransporter gene in the marine diatom Skeletonema tropicum

Abstract

Unicellular algae have evolved to express many forms of high-affinity phosphate transporters, and homologs of these proteins are broadly distributed in yeast, fungi, higher plants, and vertebrates. In this report, an effort has been made to characterize such a transporter gene, StPHO, in the marine diatom Skeletonema tropicum. The primers used for polymerase chain reaction were designed by referring to a homologous gene in a prasinophyte, and the full-length (1692 bp) cDNA of StPHO was then cloned and sequenced. Sequence alignments and secondary structure prediction indicated that StPHO is a gene that encodes a type III Na+/ Pi cotransporter (SLC20 family). To study the function of StPHO, specific concentrations of inorganic phosphate (Pi) were used to alter the physiological status of S. tropicum. In each treatment, samples were collected for the measurements of StPHO mRNA, [PO43-], cell abundance, the maximal photochemical efficiency of photosystem II (F-v/F-m), and alkaline phosphatase activity (APA). The results indicated that the ambient [PO43-] strongly affected the population growth and related physiological parameters of S. tropicum. The transcription of StPHO was fully repressed when the [PO43-] was greater than 1 mu M but increased approximately 100-fold when the ambient [PO43-] decreased to 0.02 mu M. Within this [PO43-] range, the regression equations are Y = -0.6644X + 0.9034 and Y = -0.5908X + 0.8054 for Pi-starved and Pi-limited treatments, respectively. This trend of gene expression suggested that StPHO plays an important role in the uptake of [PO43-], and StPHO may serve as a useful molecular biomarker for Pi-stressed diatom populations in marine ecosystems.