Nucleic acids encoding mutant elongation factor proteins (EF-Sep), phosphoseryl-tRNA synthetase (SepRS), and phosphoseryl-tRNA (tRNASep) and methods of use in site specific incorporation of phosphoserine into a protein or polypeptide are described. Typically, SepRS preferentially aminoacylates tRNASep with O-phosphoserine and the tRNASep recognizes at least one codon such as a stop codon. Due to the negative charge of the phosphoserine, Sept-tRNASep does not bind elongation factor Tu (EF-Tu). However, mutant EF-Sep proteins are disclosed that bind Sep-tRNASep and protect Sep-tRNASep from deacylation. In a preferred embodiment the nucleic acids are on vectors and are expressed in cells such as bacterial cells, archeaebacterial cells, and eukaryotic cells. Proteins or polypeptides containing phosphoserine produced by the methods described herein can be used for a variety of applications such as research, antibody production, protein array manufacture and development of cell-based screens for new drug discovery.