In order to identify proteins interacting with the M2 hairpin element present within the 3 0 UTR of the Pol b transcript, we performed a yeast three-hybrid screen of a rat cDNA library. The IRE-IRP interaction served as a positive control, while negative controls consisted of empty pRH5 0 and pYESTrp1 vectors and single transformants of pRH5 0 H 'bait' plasmid or of the protein-hybrid clone isolated from the library. Out of 63 positive clones obtained after the first selection, only 11 were RNA dependent, and these were sequenced and analyzed. Of these, only one was in the proper reading frame, had the correct in-frame orientation and represented a coding sequence-the terminal 622 bp of the rat Hax-1 mRNA(corresponding to the last 150 aa of the protein), ( Figure 5 ). This clone exhibited strong growth on 25 mM 3-AT medium without histidine and tested positive in the b-gal plate test (Table 1) .