Characterization of a cDNA encoding glutamine synthetase from the marine diatom Skeletonema costatum (Bacillariophyceae)

A cDNA-encoding glutamine synthetase (GS)was isolated from the marine diato m Skeletonema costatum (Greville) Cleve by PCR amplification. Nucleic acid and deduced amino acid sequences of the diatom GS were greater than 50% ide ntical to GS from green algae and vascular plants, and phylogenetic analysi s established the diatom GS as a mem ber of the GSII gene family. The prese nce of an N-terminus signal sequence, identified on the basis of sequence s imilarity with other chloroplast-localized proteins from diatoms, suggests that the encoded GS isoenzyme is localized to the chloroplast. The GS mRNA was present in log-phase cells gown with either nitrate or ammonium as the sole added nitrogen source. Results from Southern blot analysis of genomic DNA suggested that the cDNA isolated in this study was either a member of a small, highly conserved gene family or that there was allelic variation wi thin the region examined, Phylogenetic analyses further indicated that gems encoding GS from the diatom and two species of green algae diverged prior to the gene duplication, to the isoenzymes in vascular plants, supporting t he hypothesis that GS isoenzymes in diatoms, green algae, and vascular plan ts arose through independent evolutionary events.