Me. Davey et Fj. De Bruijn, A homologue of the tryptophan-rich sensory protein TspO and FixL regulate a novel nutrient deprivation-induced Sinorhizobium meliloti locus, APPL ENVIR, 66(12), 2000, pp. 5353-5359
A nutrient deprivation-induced locus in Sinorhizobium meliloti strain 1021
was identified by use of a Tn5-luxAB reporter gene transposon, The tagged l
ocus is comprised of two open reading frames (ORFs) designated ndiA and ndi
B for nutrient deprivation-induced genes A and B. Comparison of the deduced
amino acid sequences of both ndiA and ndiB to the protein databases failed
to reveal similarity to any known genes. The expression of the ndi locus w
as found to be induced by carbon and nitrogen deprivation, osmotic stress,
and oxygen limitation and during entry into stationary phase. To identify r
egulatory components involved in the control of ndi gene expression, a seco
nd round of mutagenesis was performed on the primary ndiB::Tn5-lunAB-tagged
strain (C22) with transposon Tn1721, A double-mutant strain was obtained t
hat lacked ndi locus transcriptional activity under ail of the inducing con
ditions tested. The Tn1721-tagged gene showed a high degree of similarity t
o tryptophan-rich sensory protein TspO from Rhodobacter sphaeroides, as wel
l as to mitochondrial benzodiazepine receptor pK18 from mammals. Induction
of the ndi::Tn5-luxAB reporter gene fusion was restored under all inducing
conditions by introducing the tspO coding region, from either S, meliloti o
r R sphaeroides, in trans. Furthermore, it was found that, in addition to t
spO,fixL, which encodes the sensor protein of an oxygen-sensing two-compone
nt system, is required for full expression of the ndi locus, but only under
low oxygen tension.