U RNAs are highly abundant small nuclear RNAs involved in the processing of 
messenger RNA. Most U RNA genes are thought to be transcribed by RNA polymerase 
II (pol II). However, evidence has recently been presented that U6 RNA genes are 
transcribed by RNA polymerase III (pol III). In the light of these results it 
was surprising to find that the 5' flanking region of a mouse U6 RNA gene 
includes a perfect copy of the octamer sequence motif, ATTTGCAT, found in many 
RNA polymerase II transcription enhancer elements. In the present study we show 
that deletion of mouse U6 gene sequences upstream of nucleotide position -217, 
including the octanucleotide motif, reduces U6 transcription by 90% when assayed 
in Xenopus laevis oocytes, suggesting the presence of a distant control element. 
DNase I footprinting of the 5' flanking region of the U6 gene shows protection 
of the octanucleotide sequence. Moreover, the 5' flanking sequence from -217 to 
-315 can replace the enhancer of a human U2 RNA gene. We therefore conclude that 
although U6 RNA genes appear to be transcribed by pol III, they are preceeded by 
an enhancer-like element which can functionally substitute for the enhancer of a 
pol II-transcribed U RNA gene.