Signaling via cAMP in fungi: interconnections with mitogen-activated protein kinase pathways.

The cAMP signal transduction pathway controls a wide variety of processes in fungi. For example, considerable progress has been made in describing the involvement of cAMP pathway components in the control of morphogenesis in Saccharomyces cerevisiae, Ustilago maydis, and Magnaporthe grisea. These morphological processes include the establishment of filamentous growth in S. cerevisiae and U. maydis, and the differentiation of an appressorial infection structure in M. grisea. The discovery that appressorium formation requires cAMP signaling provides an immediate connection to fungal virulence. This connection may have broader implications among fungal pathogens because recent work indicates that cAMP signaling controls the expression of virulence traits in the human pathogen Cryptococcus neoformans. In this fungus, cAMP also influences mating, as has been found for Schizosaccharomyces pombe and as may occur in U. maydis. Finally, cAMP and mitogen-activated protein kinase pathways appear to function coordinately to control the response of certain fungi, e.g., Saccharomyces cerevisiae and Schizosaccharomyces pombe, to environmental stress. There are clues that interconnections between these pathways may be common in the control of many fungal processes.

The concerted evolution of 5S ribosomal genes linked to the repeat units of other multigene families.

We review all instances in which the nuclear 5S rRNA genes of fungi, protist, nematode, and arthropod species have been reported to be linked to the tandemly repeated units of the rDNA, trans-spliced leader, and histone multigene families. The evolution of these gene arrangements is analyzed by mapping them to independently derived phylogenies. These analyses show that 5S rRNA genes have repeatedly become linked to diverse tandemly repeated gene families and that such linkages have also been subsequently inverted or lost in some species. These variable gene linkages are probably the result of stochastic gains and losses of variant repeat units, where functional 5S rRNA had transposed, by the mechanisms which are responsible for the concerted evolution of tandemly repeated multigene families. We discuss the possible mechanisms of 5S rRNA gene transposition and suggest that the characteristics of their promoter elements, transcription, and termination signals may allow functional copies of these genes to be fortuitously transposed through an RNA intermediate. We also review the evidence which shows that the linked 5S rRNA gene copies are transcribed. We conclude that the observed patterns of 5S rRNA gene linkages to the repeat units of other tandemly repeated multigene families have likely arisen due to fortuitous recombination events and are unlikely to represent the remnants of an eubacterial-like arrangement of rDNA operons or to have been established due to selective pressures.

Rapid Activation of Phenylpropanoid Metabolism in Elicitor-Treated Hybrid Poplar (Populus trichocarpa Torr. & Gray x Populus deltoides Marsh) Suspension-Cultured Cells.

Elicitor induction of phenylpropanoid metabolism was investigated in suspension-cultured cells of the fast-growing poplar hybrid (Populus trichocarpa Torr. & Gray x Populus deltoides Marsh) H11-11. Treatment of cells with polygalacturonic acid lyase or two fungal elicitors resulted in rapid and transient increases in extractable l-phenylalanine ammonia lyase and 4-coumarate:coenzyme A ligase enzyme activities. The substrate specificity of the inducible 4-coumarate:coenzyme A ligase enzyme activity appeared to differ from substrate specificity of 4-coumarate:coenzyme A ligase enzyme activity in untreated control cells. Large and transient increases in the accumulation of l-phenylalanine ammonia-lyase and 4-coumarate:coenzyme A ligase mRNAs preceded the increases in enzyme activities and were detectable by 30 minutes after the start of elicitor treatment. Chalcone synthase, cinnamyl alcohol dehydrogenase, and coniferin beta-glucosidase enzyme activities were unaffected by the elicitors, but a large and transient increase in beta-glucosidase activity capable of hydrolyzing 4-nitrophenyl-beta-glucoside was observed. Subsequent to increases in l-phenylalanine ammonialyase and 4-coumarate:coenzyme A ligase enzyme activities, cell wall-bound thioglycolic acid-extractable compounds accumulated in elicitor-treated cultures, and these cells exhibited strong staining with phloroglucinol, suggesting the accumulation of wall-bound phenolic compounds.