Catalyzes the attachment of serine to tRNA(Ser). Is also able to aminoacylate tRNA(Sec) with serine, to form the misacylated tRNA L-seryl-tRNA(Sec), which will be further converted into selenocysteinyl-tRNA(Sec). ATP + L-serine + tRNA(Ser) = AMP + diphosphate + H(+) + L-seryl-tRNA(Ser) ATP + L-serine + tRNA(Sec) = AMP + diphosphate + H(+) + L-seryl-tRNA(Sec) Binds 1 Zn(2+) ion per subunit. This ion is coordinated with 2 cysteines, 1 glutamate and a water molecule that dissociates from the zinc ion to allow the coordination of the amino group of the serine substrate, which is essential for catalysis. Thermostable. Fully active at 80 degrees Celsius. Aminoacyl-tRNA biosynthesis; selenocysteinyl-tRNA(Sec) biosynthesis; L-seryl-tRNA(Sec) from L-serine and tRNA(Sec): step 1/1. Homodimer. Consists of two distinct domains, a catalytic core and a N-terminal extension that is presumably involved in tRNA binding. Belongs to the class-II aminoacyl-tRNA synthetase family. Type-2 seryl-tRNA synthetase subfamily.