Rhizobium meliloti trc genes controlling the catabolism of trigonelline, a plant secondary metabolite often abundant in legumes, are closely linked to nif-nod genes on the symbiotic megaplasmid pSym [Boivin, C., Malpica, C., Rosenberg, C., Denarie, J., Goldman, A., Fleury, V., Maille, M., Message, B., and Tepfer, D. (1989). In Molecular Signals in the Microbe-Plant Symbiotic and Pathogenic Systems. (Berlin: Springer-Verlag), pp. 401-407]. To investigate the role of trigonelline catabolism in the Rhizobium-legume interaction, we studied the regulation of trc gene expression in free-living and in endosymbiotic bacteria using Escherichia coli lacZ as a reporter gene. Experiments performed with free-living bacteria indicated that trc genes were organized in at least four transcription units and that the substrate trigonelline was a specific inducer for three of them. Noninducing trigonelline-related compounds such as betaines appeared to antagonize the inducing effect of trigonelline. None of the general or symbiotic regulatory genes ntrA, dctB/D, or nodD seemed to be involved in trigonelline catabolism. trc fusions exhibiting a low basal and a high induced [beta]-galactosidase activity when present on pSym were used to monitor trc gene expression in alfalfa tissue under symbiotic conditions. Results showed that trc genes are induced during all the symbiotic steps, i.e., in the rhizosphere, infection threads, and bacteroids of alfalfa, suggesting that trigonelline is a nutrient source throughout the Rhizobium-legume association.

Rhizobium meliloti trc genes controlling the catabolism of trigonelline, a plant secondary metabolite often abundant in legumes, are closely linked to nif-nod genes on the symbiotic megaplasmid pSym [Boivin, C., Malpica, C., Rosenberg, C., Denarie, J., Goldman, A., Fleury, V., Maille, M., Message, B., and Tepfer, D. (1989). In Molecular Signals in the Microbe-Plant Symbiotic and Pathogenic Systems. (Berlin: Springer-Verlag), pp. 401-407]. To investigate the role of trigonelline catabolism in the Rhizobium-legume interaction, we studied the regulation of trc gene expression in free-living and in endosymbiotic bacteria using Escherichia coli lacZ as a reporter gene. Experiments performed with free-living bacteria indicated that trc genes were organized in at least four transcription units and that the substrate trigonelline was a specific inducer for three of them. Noninducing trigonelline-related compounds such as betaines appeared to antagonize the inducing effect of trigonelline. None of the general or symbiotic regulatory genes ntrA, dctB/D, or nodD seemed to be involved in trigonelline catabolism. trc fusions exhibiting a low basal and a high induced [beta]-galactosidase activity when present on pSym were used to monitor trc gene expression in alfalfa tissue under symbiotic conditions. Results showed that trc genes are induced during all the symbiotic steps, i.e., in the rhizosphere, infection threads, and bacteroids of alfalfa, suggesting that trigonelline is a nutrient source throughout the Rhizobium-legume association.

Genes controlling the catabolism of trigonelline, a secondary metabolite that is often present in legumes, are located on the pSym megaplasmid of Rhizobium meliloti. To investigate the role of bacterial trigonelline catabolism in the Rhizobium-legume symbiosis, we identified and characterized the R. meliloti RCR2011 genetic loci (trc) controlling trigoneline catabolism. Tn5-B20 mutagenesis showed that the trc region is a continuous DNA segment of 9 kb located 4 kb downstream of the nifAB and fdxN genes. Trc mutants fell into two classes according to their phenotype and location: (i) mutants carrying Tn5-B20 insertions in the right-hand part of the trc region were incapable of growing on trigonelline as the sole carbon and/or nitrogen source, and (ii) insertions in the left-hand part of the trc region resulted in delayed growth on trigonelline as the sole carbon and/or nitrogen source. No significant defect in nodule formation or nitrogen fixation was detected for mutants of either class. Screening of a set of R. meliloti strains from various geographical origins showed that all of these strains are able to catabolize trigonelline and show sequence homology between their megaplasmids and a trc probe.Among the variety of secondary metabolites synthesized by plants, some are involved in the establishment of interactions with microorganisms, as is the case for the bacteria of the family Rhizobiaceae. Agrobacterium species are pathogens which induce gall formation or root proliferations on plants (40), while the symbiotic bacteria of the genera Rhizobium, Bradyrhizobium, and Azorhizobium induce on their leguminous hosts the formation of nodules in which they fix nitrogen (25). For these two types of bacterium-plant interactions, plant secondary metabolites have been shown to play a signalling role. Monocyclic phenolic compounds present in wounded plant tissues induce the Agrobacterium genes controlling virulence (vir genes) (28). Similarly, in Rhizobium, Bradyrhizobium, and Azorhizobium species, the expression of a set of genes controlling infection and the nodulation process (nod genes) is controlled by various complex phenolic compounds of the flavonoid family (for a review, see reference 32). Moreover, other plant secondary metabolites have been shown to play a trophic role, being used as nutrient sources by members of the family Rhizobiaceae: agrobacteria utilize opines, specific plant secondary metabolites the synthesis of which is directed by genes transferred from the bacterium to the plant during infection, for growth (40). This concept has been extended to particular Rhizobium strains able to utilize rhizopines, compounds present only in the nodules of plants infected by these strains (29,34). It would seem logical that rhizobia might have evolved catabolic functions to benefit also from secondary metabolites naturally present in their hosts. This hypothesis has been supported by recent reports of the presence in * Corresponding author.

As part of a future mission to Mars, NASA is considering including a small helicopter capable of operating independently in the Martian environment. The Martian atmosphere is extremely thin, with a density of only 1-2% of Earth's atmospheric density at sea level; this significantly alters the flight dynamics of the vehicle and has implications for vehicle design and control. In this paper we focus on guidance and control for a Mars Helicopter, and in particular on the challenges that are unique to operating in the Mars environment. In 2016, the first-ever controlled flight of a helicopter in Martian atmospheric conditions was performed in the 25-ft Space Simulator at NASA's Jet Propulsion Laboratory. We provide details of the effort leading to this flight demonstration, including