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J. Bacteriol., Jun 1996, 3564-3571, Vol 178, No. 12
TJ Kenney and G Churchward
The DNA sequence of the promoter region of the Mycobacterium smegmatis rpsL
gene, which encodes the S12 ribosomal protein, was determined. Primer
extension analysis and S1 nuclease protection experiments identified the 5'
end of the rpsL mRNA to be 199 bp upstream of the translation initiation
codon. The rpsL promoter contained sequences upstream of this start point
for transcription that were similar to the canonical hexamers found at the
-10 and -35 regions of promoters recognized by Esigma70, the major form of
RNA polymerase in Escherichia coli. To define the promoter of the rpsL
gene, DNA fragments containing progressive deletions of the upstream region
of the rpsL gene were inserted into a plasmid vector containing a
promoterless xylE gene. These insertions revealed that the 200 bp of DNA
sequence immediately upstream from the translation initiation codon was not
essential for promoter function. In addition, 5' deletions removing all but
34 bp upstream of the transcription start point retained greater than 90%
promoter activity, suggesting that the -35 hexamer was not essential for
promoter activity. To determine which nucleotides were critical for
promoter function, oligonucleotide-directed mutagenesis and mutagenic PCR
amplification were used to produce point mutations in the region upstream
of the start point of transcription. Single base substitutions in the -10
hexamer, but not in the -35 hexamer, severely reduced rpsL promoter
activity in vivo. Within the -10 hexamer, nucleotide substitutions causing
divergence from the E. Coli sigma70 consensus reduced promoter activity.
The DNA sequence immediately upstream from the - 10 hexamer contained the
TGn motif described as an extended -10 region in prokaryotic promoters.
Mutations in this motif, in combination with a transition at either the -38
or -37 position within the -35 hexamer, severely reduced promoter activity,
indicating that in the absence of a functional -35 region, the rpsL
promoter is dependent on the TGn sequence upstream from the -10 hexamer.
Comparison of the nucleotide sequence of the rpsL promoter region of M.
smegmatis with the homologous sequences from Mycobacterium leprae,
Mycobacterium bovis, and Mycobacterium tuberculosis showed the presence in
these slowly growing mycobacterial species of conserved promoter elements a
similar distance upstream of the translation initiation codon of the rpsL
gene, but these other mycobacterial promoters did not contain the extended
-10 motif.
Copyright © 1996, American Society for Microbiology
Genetic analysis of the Mycobacterium smegmatis rpsL promoter
Department of Microbiology and Immunology, Emory University, Atlanta, Georgia 30322, USA.
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