Abstract
Transcription initiation has been shown to occur
in vitro at several sites within a cloned
Caulobacter crescentus ribosomal RNA gene cluster that lacks the major promoter region 5′ to the 16 S rRNA gene. The predominant transcription start site
in vitro was located near the 3′ end of the 16 S rRNA gene. Transcription initiation from this region was also detected
in vivo, when the cloned rRNA gene cluster was present on a multi-copy plasmid. The transcription start sites
in vitro and
in vivo were shown to be identical by S
1 nuclease mapping and were found to be located approximately 300 nucleotides upstream from the 3′ end of the 16 S rRNA gene. The transcript synthesized
in vitro was shown to be cleaved by
C. crescentus RNase III and to release the transfer RNA genes from the downstream 16 S/23 S intergenic spacer region. Analysis of the nucleotide sequence near the internal 16 S rRNA transcription start site revealed the presence of a consensus promoter sequence followed by the beginning of an open reading frame approximately 90 nucleotides downstream. Examination of the 16 S rRNA genes from other bacterial species and chloroplasts and 18 S rRNA genes from
Xenopus and yeast revealed that the nucleotide sequence of this internal 16 S rRNA promoter region was highly conserved. Although the length of these 16 S and 18 S rRNA genes is slightly variable, the distance of the conserved promoter sequence from the 3′ end of these genes has been conserved.