The Trithorax-mimic allele of Enhancer of zeste renders active domains of target genes accessible to polycomb-group-dependent silencing in Drosophila melanogaster.

Two antagonistic groups of genes, the trithorax- and the Polycomb-group, are proposed to maintain the appropriate active or inactive state of homeotic genes set up earlier by transiently expressed segmentation genes. Although some details about the mechanism of maintenance are available, it is still unclear how the initially active or inactive chromatin domains are recognized by either the trithorax-group or the Polycomb-group proteins. We describe an unusual dominant allele of a Polycomb-group gene, Enhancer of zeste, which mimics the phenotype of loss-of-function mutations in trithorax-group genes. This mutation, named E(z)(Trithorax mimic) [E(z)(Trm)], contains a single-amino-acid substitution in the conserved SET domain. The strong dominant trithorax-like phenotypes elicited by this E(z) allele suggest that the mutated arginine-741 plays a critical role in distinguishing between active and inactive chromatin domains of the homeotic gene complexes. We have examined the modification of E(z)(Trm) phenotypes by mutant alleles of PcG and trxG genes and other mutations that alter the phosphorylation of nuclear proteins, covalent modifications of histones, or histone dosage. These data implicate some trxG genes in transcriptional repression as well as activation and provide genetic evidence for involvement of histone modifications in PcG/trxG-dependent transcriptional regulation.

Mutation in the ntrR gene, a member of the vap gene family, increases the symbiotic efficiency of Sinorhizobium meliloti.

In specific plant organs, namely the root nodules of alfalfa, fixed nitrogen (ammonia) produced by the symbiotic partner Sinorhizobium meliloti supports the growth of the host plant in nitrogen-depleted environment. Here, we report that a derivative of S. meliloti carrying a mutation in the chromosomal ntrR gene induced nodules with enhanced nitrogen fixation capacity, resulting in an increased dry weight and nitrogen content of alfalfa. The efficient nitrogen fixation is a result of the higher expression level of the nifH gene, encoding one of the subunits of the nitrogenase enzyme, and nifA, the transcriptional regulator of the nif operon. The ntrR gene, controlled negatively by its own product and positively by the symbiotic regulator syrM, is expressed in the same zone of nodules as the nif genes. As a result of the nitrogen-tolerant phenotype of the strain, the beneficial effect of the mutation on efficiency is not abolished in the presence of the exogenous nitrogen source. The ntrR mutant is highly competitive in nodule occupancy compared with the wild-type strain. Sequence analysis of the mutant region revealed a new cluster of genes, termed the "ntrPR operon," which is highly homologous to a group of vap-related genes of various pathogenic bacteria that are presumably implicated in bacterium-host interactions. On the basis of its favorable properties, the strain is a good candidate for future agricultural utilization.

Transduction of plant signal molecules by the Rhizobium NodD proteins.

The regulatory NodD proteins of Rhizobium bacteria mediate the activation of a gene set responsible for symbiotic nodule formation by plant signal molecules. Here we discuss the signal recognition and gene activation properties of NodD and present a model summarizing the current knowledge on NodD action.

Homology of the ligand-binding regions of Rhizobium symbiotic regulatory protein NodD and vertebrate nuclear receptors.

The signal specificity and structure of sensor-activator proteins from different species (NodD of Rhizobium bacteria and vertebrate nuclear receptors) were compared. Several compounds (including flavonoids, coumestrol and estradiol) that bind to mammalian receptors also interact with NodD proteins. NodD-dependent synergism of the signal compounds luteolin and catechin was observed suggesting that these compounds bind directly to NodD. Two regions comprising 63 and 37 amino acids in NodD showed 45% and 36% homology, respectively, with the estrogen receptor. These regions, designated as modules M1 and M2, coincide with conserved parts of the ligand-binding domains of the nuclear receptors. A part of NodD overlapping with the M1 module was predicted to be membrane associated and was 46% homologous to a membrane-spanning sensory segment of the Agrobacterium VirA protein. We suggest that the homologous polypeptide modules detected in NodD and the nuclear receptors originate from a common ancestor protein and may be directly involved in ligand binding.

Organization, structure and symbiotic function of Rhizobium meliloti nodulation genes determining host specificity for alfalfa.

In R. meliloti we have identified four nodulation genes determining plant host-range specificity and have designated them hsnABC and D. The genes code for 9.7, 41.7, 26.7, and 28.6 kd proteins, respectively, and are organized into two transcriptional units. Mutations in these genes affect nodulation of their natural plant hosts Medicago sativa and Melilotus albus to different extents and hsnD mutants have an altered host-range. These Nod- mutations are not complementable by nodulation genes of other Rhizobium species such as R. leguminosarum. The hsn genes determine plant-specific infection through root hairs: hsnD is required for host-specific root hair curling and nodule initiation while the hsnABC genes control infection thread growth from the root hairs.