Silvia Rossbach | PubMed Health

PubMed2006

A highly conserved Sinorhizobium meliloti operon is induced microaerobically via the FixLJ system and by nitric oxide (NO) via NnrR.

A previously generated collection of 11 Tn5-luxAB insertion mutants of Sinorhizobium meliloti harbouring lux reporter gene fusions induced under microaerobic (1% O2) conditions was further characterized and mapped on the sequenced S. meliloti genome. One highly induced gene fusion from this collection (loe-7) was found to be located in the intergenic region between sma1292, encoding a putative protease/collagenase, and a gene of unknown function (sma1294). The loe-7 fusion had been shown previously to be partially controlled by the oxygen sensor/regulator FixLJ system, but significant ( approximately 40%) Lux activity remained in a fixLJ mutant background. Therefore, a secondary Tn1721 mutagenesis of the loe-7 strain was carried out. Nine Tn1721 ('dark') insertions completely abolishing the Lux activity of the loe-7 fusion under microaerobic conditions were isolated. Surprisingly, five dark insertions mapped in denitrification genes [napA, napC, nirK--two insertions--and sma1245 encoding a NnrR-like transcriptional regulator controlling denitrification in response to nitric oxide (NO)]; Tn1721 insertions in the respiration genes fixG and fixP resulted in a reduced expression of the loe-7-lux fusion, and insertions in the regulatory genes fixJ and fixK1 resulted in low, but still detectable Lux activity. On the contrary, insertions in the norD or norQ genes resulted in constitutive Lux activity. In these mutant strains, NO would be expected to accumulate under microaerobic conditions. NO was found to be able to strongly induce the loe-7-luxAB fusion under microaerobic and aerobic conditions, but only in the presence of the functional nnrR-like gene (sma1245). These results suggest that NO, via the NnrR regulator, can serve as a signal molecule to induce the loe-7-luxAB fusion in concert with the FixLJ system.

Bacterial Proteins↗

PubMed2002

Engineering bacterial competitiveness and persistence in the phytosphere.

Several tactics exist to improve the survival of an introduced microorganism of interest in the plant environment. One, derived from studies on the Agrobacterium-plant interaction and the role of opines in this interaction, proposes to promote growth of the inoculant in the plant environment via the establishment of a bias in the rhizosphere. It is supported by the occurrence of natural biases, such as those generated by opine-like molecules, by calestegins, or by mimosine. Opine-mediated biases have allowed several investigators to favor the growth of opine-degrading bacteria or communities under sterile or axenic environments or in microcosms mimicking near field conditions. Another way to favor a given microbe consists in impeding growth of competing microorganisms. Experiments performed using detergent or bacteriostatic agents as amendments under field or near field conditions yielded promising results. Research perspectives for engineering plant-microbe interactions also include specific engineering of predation and strategies designed to interfere with some of the signals perceived by the microbes, provided these signals control the expression of functions central to microbial fitness. In this respect, quorum-sensing signal molecules, such as N-acyl-homoserine lactones, may be valuable targets for the development of biocontrol agents and procedures.

Bacteria↗