Human MnSOD localizes to the mitochondria and plays a key protective role by detoxifying oxygen free radicals. The MnSOD mRNA 3' UTR contains a 280-bp region (Alu-like element or Alu-E) that shows high homology to human Alu and 7SL sequences. MnSOD 3' UTR probes hybridize to a specific cytoplasmic RNA species of similar to 300 nucleotides. This antisense RNA is most Likely 7SL RNA based on its size, ubiquitousness, high levels, and lack of inducibility, Hybridization of this small RNA to the MnSOD 3' UTR may modulate posttranscriptional MnSOD gene expression. This regulation could occur by several means including inhibition of translation and mRNA destabilization, Regulation at the level of translational initiation does not seem to occur as MnSOD mRNA containing the Alu-E is efficiently bound by ribosomes, To test the role of the MnSOD 3' UTR, and in particular the Alu-E in gene expression, luciferase reporter gene constructs were made containing various regions of the MnSOD 3' UTR including the Alu-E, These constructs were transfected into human A549 lung carcinoma cells and luciferase activity was measured. Reporter constructs containing the MnSOD 3' UTR and the Alu-E repress luciferase activity. Taken together, these results suggest that naturally occurring antisense RNA may bind MnSOD mRNA and repress its expression. These results also suggest that other mRNAs containing Alu elements may be similarly repressed,