Clones containing variant forms of complete human rRNA genes. Characterization and sequence of their transcription initiation region.

In order to assess the extent of structural heterogeneity among ribosomal genes in the human genome, several cosmid clones, each containing an entire transcription unit, have been isolated and analyzed. Five cloned genes were recognized as structurally different from each other to some extent, either within the transcription unit or in its immediate vicinity. The sequence of a 1.2 kb region encompassing the transcription start site has been determined for the five cloned genes. Point differences among the genes are observed at five nucleotide positions within the analyzed portion of the 5' external transcribed spacer of the ribosomal gene. Moreover, upstream from the transcription start site, a unique difference is observed with the absence from the three genes of a 19-nucleotide long stretch which is present in the two other variant gene forms.