To examine the RNA polymerase (EC 2.7.7.6) specificity of RNA 
maturation/utilization and transcriptional enhancement, we constructed a 
chimeric plasmid (pPolI-CAT) in which a promoter for mouse rRNA gene 
transcription was placed adjacent the coding sequences for chloramphenicol 
acetyltransferase (CAT; EC 2.3.1.28). A number of other constructs, including 
plasmids also containing a murine sarcoma virus enhancer or lacking any natural 
eukaryotic promoter sequences, were also prepared. In apparent agreement with 
earlier conclusions that an RNA polymerase I transcript can act as a messenger 
RNA, transient transfection of mouse L cells with pPolI-CAT yielded both high 
levels of transcription from the RNA polymerase I promoter and enzymatically 
active CAT protein. However, further examination revealed that CAT protein is 
not translated from RNA that begins at the normal rRNA transcription initiation 
site. Polysomal RNA is devoid of such RNA and instead consists of CAT-encoding 
transcripts that begin elsewhere in the mouse ribosomal DNA (rDNA) region. Since 
transcription of these aberrant RNAs is stimulated by the addition of a murine 
sarcoma virus enhancer segment, they are probably transcribed by RNA polymerase 
II. Transcripts that map to the authentic rRNA start site are not similarly 
enhanced. Moreover, unlike the RNAs deriving from the rRNA initiation site, 
these aberrant RNAs are more stable and the level of translatable CAT 
transcripts is suppressed by inclusion of larger segments of the rDNA promoter 
regions. Fortuitously initiated mRNAs are also formed in the absence of any 
natural eukaryotic promoter sequence. From these data we conclude that there is 
no evidence that normal RNA polymerase I transcription yields functional mRNA 
and that transcriptional enhancement appears to be RNA polymerase specific.