Rhizobium fix genes mediate at least two communication steps in symbiotic nodule development.

To identify bacterial genes involved in symbiotic nodule development, ineffective nodules of alfalfa (Medicago sativa) induced by 64 different Fix-mutants of Rhizobium meliloti were characterized by assaying for symbiotic gene expression and by morphological studies. The expression of leghemoglobin and nodulin-25 genes from alfalfa and of the nifHD genes from R. meliloti were monitored by hybridizing the appropriate DNA probes to RNA samples prepared from nodules. The mutants were accordingly divided into three groups. In group I none of the genes were expressed, in group II only the plant genes were expressed and in group III all three genes were transcribed. Light and electron microscopical analysis of nodules revealed that nodule development was halted at different stages in nodules induced by different group I mutants. In most cases nodules were empty lacking infection threads and bacteroids or nodules contained infection threads and a few released bacteroids. In nodules induced by a third mutant class bacteria were released into the host cells, however the formation of the peribacteroid membrane was not normal. On this basis we suggest that peribacteroid membrane formation precedes leghemoglobin and nodulin-25 induction, moreover, after induction of nodulation by the nod genes at least two communication steps between the bacteria and the host plants are necessary for the development of the mature nodule. By complementing each mutant of group I with a genomic R. meliloti library made in pLAFRl, four new fix loci were identified, indicating that several bacterial genes are involved in late nodule development.

On the road to PACS--confronting the issues.

In 1992, PACS became a serious consideration for me, the new administrative director of medical imaging, and a team approach seemed the sensible means to a topic so large. Even with our limited knowledge of PACS at that time, the team knew it would take years to deploy a PACS system fully. Members realized it would be one thing to design a complete PACS and quite another to break the whole into functional, logical and compatible parts. Yet how could we plan for something so sophisticated when we couldn't even agree where to begin? The team, recognizing the financial challenges of calculating cost/benefits, finally determined that the cost of PACS would exceed obvious savings. Ultimately, we determined that PACS efficiencies would enable us to go after new business. But were there hidden costs? How could we balance costs with benefits that are seemingly valid but hard to quantify? To answer such questions, the team broke the process into steps and planned each part of every step. Implications for PACS outside the radiology department became an issue. How would the PACS, RIS and transcription systems work together to handle images and reports? What about referring physicians? Will patient care improve? After inviting major image-printing vendors to make presentations for a print network, the team selected a firm endorsed by our information technology services department. Where once there was confusion, the team accomplished its goals of reducing costs, improving efficiency, and maintaining high standards for patient care.

Celebration: AHRA (American Healthcare Radiology Administrators) 15th anniversary. Perspectives.

The seven papers in this section present individual viewpoints on the past 15 years, with a great variety in emphasis and perspective. Several authors take a nostalgic look back, one describes an overseas experience, and another provides a business outlook. Read and enjoy these contributions from your colleagues.

[Cossio syndrome: above average life expectancy of a patient with congenital large atrial septal defect]. / Cossio-Syndrom: Uberdurchschnittliche Uberlebenszeit bei angeborener grosser Vorhoflücke.

Cossio's syndrome is demonstrated in a patient with a congenital large atrial septal defect who died at the age of 72. The clinical and pathomorphological findings as well as the pathophysiological implications on pulmonary circulation are described. The probable genesis of this interauricular defect is discussed.

Differential transcript induction of parsley pathogenesis-related proteins and of a small heat shock protein by ozone and heat shock.

Parsley (Petroselinum (crispum L.) is known to respond to pathogen attack by the synthesis of furanocoumarins and to UV irradiation by the synthesis of flavone glycosides whereas ozone treatment results in the induction of both pathways. A cDNA library from parsley plants was differentially screened using labelled reverse-transcribed poly(A)+ RNA isolated from ozone-treated parsley plants. This resulted in the isolation of 13 independent cDNA clones representing ozone-induced genes and of 11 cDNA clones representing ozone-repressed genes. DNA sequencing of several clones resulted in the identification of pathogenesis-related protein 1-3 (PR1-3), of a new member of PR1 cDNAs (PRI-4) and of a small heat shock protein (sHSP). Northern blot analyses showed a transient induction of the three mRNA species after ozone fumigation. In contrast, heat shock treatment of parsley plants resulted in an increase of sHSP mRNA whereas no increase for transcripts of PR1-3 and PR1-4 could be observed. This is the first characterized sHSP cDNA clone for plants induced by heat shock, as well as by oxidative stress caused by ozone.

The pha gene cluster of Rhizobium meliloti involved in pH adaptation and symbiosis encodes a novel type of K+ efflux system.

The fix-2 mutant of Rhizobium meliloti affected in the invasion of alfalfa root nodules (Inf-/Fix-) is K+ sensitive and unable to adapt to alkaline pH in the presence of K+. Using directed Tn5 mutagenesis, we delimited a 6kb genomic region in which mutations resulted in both Inf-/Fix- and K+-sensitive phenotypes. In this DNA region, seven open reading frames (ORFs) were identified and the corresponding genes were designated phaA, B, C, D, E, F and G. The putative PhaABC proteins exhibit homology to the subunits of a Na+/H+ antiporter from an alkalophilic Bacillus strain. Moreover, PhaA and PhaD also show similarity to the ND5 and ND4 subunits of the proton-pumping NADH:ubiquinone oxidoreductase respectively. Computer analysis suggests that all seven proteins are highly hydrophobic with several possible transmembrane domains. Some of these domains were confirmed by generating active alkaline phosphatase fusions. Ion transport studies on phaA mutant cells revealed a defect in K+ efflux at alkaline pH after the addition of a membrane-permeable amine. These results suggest that the pha genes of R. meliloti encode for a novel type of K+ efflux system that is involved in pH adaptation and is required for the adaptation to the altered environment inside the plant.

Rhizobium meliloti lipopolysaccharide and exopolysaccharide can have the same function in the plant-bacterium interaction.

A fix region of Rhizobium meliloti 41 involved both in symbiotic nodule development and in the adsorption of bacteriophage 16-3 was delimited by directed Tn5 mutagenesis. Mutations in this DNA region were assigned to four complementation units and were mapped close to the pyr-2 and pyr-29 chromosomal markers. Phage inactivation studies with bacterial cell envelope preparations and crude lipopolysaccharides (LPS) as well as preliminary characterization of LPS in the mutants indicated that these genes are involved in the synthesis of a strain-specific LPS. Mutations in this DNA region resulted in a Fix- phenotype in AK631, an exopolysaccharide (EPS)-deficient derivative of R. meliloti 41; however, they did not influence the symbiotic efficiency of the parent strain. An exo region able to restore the EPS production of AK631 was isolated and shown to be homologous to the exoB region of R. meliloti SU47. By generating double mutants, we demonstrated that exo and lps genes determine similar functions in the course of nodule development, suggesting that EPS and LPS may provide equivalent information for the host plant.