S. Dabizzi et al., Expression of horizontally transferred gene clusters: activation by promoter-generating mutations, RES MICROB, 152(6), 2001, pp. 539-549
The occurrence of promoter-generating mutations allowing the transcription
of heterologous genes has been studied in a system based on the plasmid-med
iated conjugal transfer of histidine biosynthetic genes from a donor bacter
ium (Azospirillum brasilense) into a heterologous Escherichia coli mutant p
opulation lacking histidine biosynthetic ability and initially unable to re
cognize the transcriptional signal of the introgressed. gene(s). Under sele
ctive stressful conditions, His(+) revertants accumulated in the E. coli Hi
s(-) culture. The number of His(+) colonies was dependent on the time of in
cubation under selective conditions, the strength of selective pressure, an
d on the crowding of cells plated; moreover, it was independent of the phys
iological status of the cell (i.e. the growth phase). Sequence analysis of
plasmid DNA extracted from E. coli His(+) revertants revealed that single b
ase substitutions in the region upstream of the A. brasilense his operon. r
esulted in an adjustment of the pre-existing sequence that was rendered sim
ilar to the E. coli - 10 promoter sequence and transcriptable by the host R
NA-polymerase. One particular transition (C --> T) was predominant in the H
is(+) revertants. Data presented here indicated that the barriers to the ex
pression of horizontally transferred heterologous genes or operons may be o
vercome in a short time scale and at high frequency, and supported the self
ish operon model on the origin and evolution of gene clusters. (C) 2001 Edi
tions scientifiques et medicales Elsevier SAS.