[Rpb1 and Rpb2] Mapping of the contact sites on two large subunits of the fission yeast Schizo-saccharomyces pombe RNA polymerase II with two small subunits,Rpb3 and Rpb5,was carried out using the two-hybrid screening system in the budding yeast Saccharomyces cerevisiae.Rpb5 was found to interact with any fragment of Rpb1 that contained the region H,which is conserved among the subunit 1 homologues of all RNA polymerases,including the β' subunit of prokaryotic RNA polymerases.In agreement with the fact that Rpb5 is shared among all three forms of eukaryotic RNA polymerases,the region H of RNA polymerase I subunit 1 (Rpa190) was also found to interact with Rpb5.On the other hand,two-hybrid screening of Rpb2 fragments from RNA polymerase II indicated the presence of an Rpb3 contact site in the region H which is conserved among the subunit 2 homologues of all RNA polymerases,including the b subunit of prokaryotic RNA polymerases.Possible functions of the regions H in the subunits 1 and 2 are discussed.