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Microarray Analysis of the Mycobacterium tuberculosis Transcriptional Response to the Acidic Conditions Found in Phagosomes. Mark A. Fisher, 2002.We used microarrays and real-time reverse transcription-PCR to analyze the global transcriptional response of Mycobacterium tuberculosis to low pH in vitro, which may mimic an environmental signal encountered by phagocytosed mycobacteria . Eighty-one genes were differentially expressed >1.5-fold, including many involved in fatty acid metabolism . The most highly induced genes showed homology with nonribosomal peptide synthetases/polyketide synthases . Transcription-Dependent Increase in Multiple Classes of Base Substitution Mutations in Escherichia coli. Joanna Klapacz, 2002.We showed previously that transcription in Escherichia coli promotes C · G-to-T · A transitions due to increased deamination of cytosines to uracils in the nontranscribed but not the transcribed strand (A . Beletskii and A . S . Bhagwat, Proc . Natl . Acad . Sci . USA 93:13919-13924, 1996) . To study mutations other than that of C to T, we developed a new genetic assay that selects only base substitution mutations and additionally excludes C · G to T · A transitions . This novel genetic reversion system is based on mutations in a termination codon and involves positive selection for resistance to bleomycin or kanamycin . Using this genetic system, we show here that transcription from a strong promoter increases the level of non-C-to-T as well as C-to-T mutations . We find that high-level transcription increases the level of non-C-to-T mutations in DNA repair-proficient cells in three different sequence contexts in two genes and that the rate of mutation is higher by a factor of 2 to 4 under these conditions . These increases are not caused by a growth advantage for the revertants and are restricted to genes that are induced for transcription . In particular, high levels of transcription do not create a general mutator phenotype in E . coli . Sequence analysis of the revertants revealed that the frequency of several different base substitutions increased upon transcription of the bleomycin resistance gene and that G · C-to-T · A transversions dominated the spectrum in cells transcribing the gene . These results suggest that high levels of transcription promote many different spontaneous base substitutions in E . coli .
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