Our approach to testing this is exemplified above in the case of Mth687. The genes, Mth687, Mth688, Mth11, and Mth1618 were cloned and expressed in E. coli as his-tagged fusions. Getting these expressed in E. coli required some finagling, including the now-routine coexpression of tRNAs that decode codons that are frequent in these genes but poorly translated in E. coli. The recombinant proteins were purifed and used to make polyclonal antisera in rabbits. These sera were used in western blots to show that these proteins copurify with RNase P activity, and in northern blots to show that they copurify with RNase P RNA. As you can see in the example above, a band of the expected size lights up in the western blot using the Mth687 antiserum in exactly those fractions of the partially purified RNase P that are the most catalytically active.