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Applied and Environmental Microbiology, March 2001, p. 1308-1317, Vol. 67, No. 3
Department of Plant and Microbial Biology,
University of California, Berkeley, California 94720
Received 16 June 2000/Accepted 21 December 2000
A set of three sucrose-regulated transcriptional fusions was
constructed. Fusions p61RYTIR, p61RYlac, and p61RYice contain the
scrR sucrose repressor gene and the promoterless
gfp, lacZ, and inaZ
reporter genes, respectively, fused to the scrY promoter from Salmonella enterica serovar Typhimurium. Cells of
Erwinia herbicola containing these fusions are
induced only in media amended with sucrose, fructose, or sorbose. While
a large variation in sucrose-dependent reporter gene activity was
observed in cells harboring all gene fusions, fusions to the
inaZ reporter gene yielded a much wider range of
activity and were responsive to lower levels of sucrose than either
lacZ or gfp. The lacZ
reporter gene was found to be more efficient than gfp,
requiring approximately 300-fold fewer cells for a detectable response
over all concentrations of sucrose. Similarly, inaZ was
found to be more efficient than lacZ, requiring 30-fold
fewer cells at 1.45 µM sucrose and 6,100-fold fewer cells at 29 mM
sucrose for a quantifiable response. The fluorescence of individual
cells containing p61RYTIR was quantified following epifluorescence
microscopy in order to relate the fluorescence exhibited by populations
of cells in batch cultures with that of individual cells in such
cultures. While the mean fluorescence intensity of a population of
individual cells increased with increasing concentrations of sucrose, a
wide range of fluorescence intensity was seen among individual cells.
For most cultures the distribution of fluorescence intensity among
individual cells was log-normally distributed, but cells grown in
intermediate concentrations of sucrose exhibited two distinct
populations of cells, one having relatively low fluorescence and
another with much higher fluorescence. When cells were
inoculated onto bean leaves, whole-cell ice nucleation and
gfp-based biological sensors for sucrose each indicated
that the average concentration of sucrose on moist leaf surfaces was about 20 µM. Importantly, the variation in green fluorescent protein fluorescence of biosensor cells on leaves suggested that large spatial
variations in sugar availability occur on leaves.
0099-2240/01/$04.00+0 DOI: 10.1128/AEM.67.3.1308-1317.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
Biological Sensor for Sucrose Availability:
Relative Sensitivities of Various Reporter Genes


*
Corresponding author. Mailing address: Department of
Plant and Microbial Biology, 111 Koshland Hall, University of
California, Berkeley, CA 94720. Phone: (510) 642-4174. Fax: (510)
642-4995. E-mail: icelab{at}socrates.berkeley.edu.
Present address: United States Department of Agriculture,
Agricultural Research Service, Western Regional Research Center, Food
Safety and Health Research Unit, Albany, CA 94710.
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