The influence of the M2 hairpin structure within the short 3 0 UTR of Pol b mRNA on expression was analyzed utilizing a luciferase reporter system. The rat hepatoma cell line FTO-2B was transfected with reporter constructs bearing the Firefly luciferase gene appended by the Pol b 3 0 UTR sequence containing the M2 hairpin element, (pCMLucH) and by the same sequence containing a structure-disrupting mutation in the hairpin-forming region (pCMLucHmut). A vector bearing the unmodified luciferase gene served as a control (pCMLuc). To asses mRNA levels of the reporter, quantitative PCR was performed for cDNA preparations obtained from cells subjected to the same transfections as for luciferase assays. The results (Figure 4) show a significant decrease of mRNA levels for the construct with the hairpin structure (H), compared with almost unchanged mRNA levels for the mutated hairpin (Hmut). These differences, however, are not reflected at the protein level: both constructs (H and Hmut) caused an increase in luciferase expression, although only for the Hmut is this increase significant. The relative increase of protein levels in respect to mRNA levels is therefore more than 3-fold for the construct with the intact hairpin, while only a slight relative increase ($1.7-fold) was observed for the mutated construct. These changes in expression indicate a complex posttranscriptional regulation in which the hairpin element has a key function.