We have cloned and sequenced a 977bp DNA fragment, pXTU6-2, that represents the 
transcription unit for a Xenopus tropicalis U6 RNA gene. This basic repeating 
unit is reiterated ca.500-fold per haploid genome. Oocyte injections of pXTU6-2 
led to the transcription of a mature-sized U6 RNA that, however, lacked internal 
2'-O-methylations. These posttranscriptional modifications of U6 RNA might be 
cytoplasmic and could require its association with U4 RNA to be accomplished. 
The low alpha- amanitin sensitivity of U6 RNA synthesis in oocytes suggested 
that U6 RNA is transcribed by RNA polymerase III, consistent with features of 
the U6 RNA molecule which also contains a Box A- like intragenic control region. 
Inspection of X. tropicalis, mouse and human U6 DNA upstream sequences revealed 
the presence of a TATA box as well as of the proximal and enhancer (octamer 
motif) elements contained in snRNA genes transcribed by RNA polymerase II. We 
propose that U6 RNAs are synthesized by a specialized transcription complex 
consisting of RNA polymerase III and transcription factors, some of which are 
very likely shared with RNA polymerase II promoters.