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Applied and Environmental Microbiology, June 2001, p. 2538-2544, Vol. 67, No. 6
0099-2240/01/$04.00+0 DOI: 10.1128/AEM.67.6.2538-2544.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
Chromosomal Gene Inactivation in the Green Sulfur
Bacterium Chlorobium tepidum by Natural
Transformation
Niels-Ulrik
Frigaard* and
Donald A.
Bryant
Department of Biochemistry and Molecular
Biology, The Pennsylvania State University, University Park,
Pennsylvania
Received 12 December 2000/Accepted 18 March 2001
Conditions for inactivating chromosomal genes of Chlorobium
tepidum by natural transformation and homologous recombination were established. As a model, mutants unable to perform nitrogen fixation were constructed by interrupting nifD with
various antibiotic resistance markers. Growth of wild-type C.
tepidum at 40°C on agar plates could be completely inhibited
by 100 µg of gentamicin ml
1, 2 µg of erythromycin
ml
1, 30 µg of chloramphenicol ml
1, or 1 µg of tetracycline ml
1 or a combination of 300 µg of
streptomycin ml
1 and 150 µg of spectinomycin
ml
1. Transformation was performed by spotting cells and
DNA on an agar plate for 10 to 20 h. Transformation frequencies on
the order of 10
7 were observed with gentamicin and
erythromycin markers, and transformation frequencies on the order of
10
3 were observed with a
streptomycin-spectinomycin marker. The frequency of spontaneous
mutants resistant to gentamicin, erythromycin, or
spectinomycin-streptomycin was undetectable or significantly lower than
the transformation frequency. Transformation with the gentamicin marker
was observed when the transforming DNA contained 1 or 3 kb of total
homologous flanking sequence but not when the transforming DNA
contained only 0.3 kb of homologous sequence. Linearized plasmids
transformed at least an order of magnitude better than circular
plasmids. This work forms a foundation for the systematic targeted
inactivation of genes in C. tepidum, whose 2.15-Mb
genome has recently been completely sequenced.
*
Corresponding author. Mailing address: Department of
Biochemistry and Molecular Biology, 232 S. Frear Building, The
Pennsylvania State University, University Park, PA 16802. Phone: (814)
863-7405. Fax: (814) 863-7024. E-mail: nxf10{at}psu.edu.
Applied and Environmental Microbiology, June 2001, p. 2538-2544, Vol. 67, No. 6
0099-2240/01/$04.00+0 DOI: 10.1128/AEM.67.6.2538-2544.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
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