35S pulse Labelling of Chlamydomonas Chloroplast Proteins 35S pulse labelling of proteins is used to attach a radioactive label to newly synthesized proteins, as sulfur is an element that is mainly present in proteins (Fleischmann and Rochaix 1999). Depending on your organism’s uptake mechanisms you need cysteine, methionine or sulfuric acid as a source of radioactive sulfur. This example uses Chlamydomonas cells and H235SO4 (Schwarz et al., 2012).