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Cellulase gene transcription in Cellulomonas fimi and an Agrobacterium Greenberg, Norman Michael


Transcriptional analysis was used to investigate the molecular mechanisms which effect cellulase gene expression in the gram-positive bacterium Cellulomonas fimi strain ATCC 484 and the gram-negative bacterium Agrobacterium sp. strain ATCC 21400. The cenA, cex and cenB genes of C. fimi encoding the extracellular β-1,4-endoglucanase, EngA (EC; Mr 48,700), the extracellular β-1, 4-exoglucanase, Exg (EC; Mr 47,300) and the extracellular β-1,4-endoglucanase EngB (EC; Mr 110,000) respectively, were characterised. By northern blot analysis, cenA mRNA was detected in C. fimi RNA prepared from glycerol- and carboxymethylcellulose (CMC)-grown cells but not in RNA from glucose-grown cells. The cex mRNA was found only in RNA from CMC-grown cells. The cenB mRNA was found in all three preparations of RNA. Therefore, the expression of these genes is subject to regulation by the carbon source provided to C. fimi. High resolution nuclease SI protection studies with unique 5'-labeled DNA probes and C. fimi RNA isolated in vivo, were used to map the 5' termini of cenA and cex mRNAs. Two cenA mRNA 5' ends, 11 bases apart, mapped 51 and 62 bases upstream of the cenA start codon, suggesting that in vivo, cenA transcription was directed from two promoters in tandem. The cex mRNA 5' end was found to map 28 bases upstream of the cex start codon. Using SI mapping with unlabeled DNA probes and C. fimi RNA which had been isolatedin vivo but which had been 5'-labeled in vitro with vaccinia virus capping enzyme confirmed that true transcription initiation sites for cenA and cex mRNA had been identified. The SI mapping revealed mRNA 3' termini 1,438, 1,449, and 1, 464 bases from the major cenA start site, and one 3' terminus 1,564 bases from the major cex mRNA start site, in good agreement with the northern blot data. High resolution SI studies were also used to show that abundant mRNA 5' ends mapped upstream of the cenB start codon in RNA prepared from CMC-grown cells, while less-abundant species mapped 52 bases closer to the ATG codon in RNA prepared from C. fimi grown on any one of the three substrates. These results seem to indicate a tandem promoter arrangement with an ATG-proximal promoter directing low-level constitutive cenB transcription and a more distal promoter directing higher levels of cenB transcription as a result of C. fimi growth on cellulosic substrate. Steady- state levels were determined for cenA, cex and cenB mRNAs with RNA prepared from glycerol-, glucose-, and CMC-grown cultures of C. fimi in slot-blot hybridisations with radiolabeled oligodeoxyribonucleotide probes. A cex-linked gene (clg) was identified by sequence inspection and SI mapping. Transcripts of the abg gene encoding the β-glucosidase (Abg, EC Mr 50,000) of Agrobacterium sp. strain ATCC 21400 were also characterised. Northern blot analysis of Agrobacterium RNA revealed the size of the in vivo abgmRNA was approximately 1,500 bases in length. High resolution SI mapping determined abg mRNA 5' ends 22 bases upstream of the abg ATG codon and 3' ends 71 bases downstream of the abg stop codon.

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