Expression of Erwinia amylovora hrp genes in response to environmental stimuli.

Seven hrp loci that are essential for the hypersensitive reaction elicited by Erwinia amylovora were transcriptionally fused with a derivative of transposon Tn5, containing the promoterless Escherichia coli beta-glucuronidase reporter gene. The seven hrp fusions were used to monitor expression of the hrp loci in vitro and in planta. No significant expression was detected in rich medium for any of the fusions. However, five of them were expressed highly in planta and in inducing medium that contains mannitol, salts, and 5 mM (NH4)2SO4. Expression of these five hrp loci is regulated by ammonium, nicotinic acid, complex-nitrogen sources, certain carbon sources, temperature, and pH. Under well-defined conditions, i.e., in inducing medium, no specific plant components were required for transcriptional activation of the hrp loci. The high levels of expression detected in vitro were comparable to those determined during the development of the hypersensitive reaction in tobacco. Differential levels of expression of the hrp loci occurred in host and nonhost plants. In pear, a host plant, expression of the hrp loci was delayed and greatly reduced compared with expression in tobacco leaves, a nonhost.

Nucleotide sequence and structural analysis of the zeste locus of Drosophila melanogaster.

The zeste locus plays a central role in transvection phenomena, where the synaptic pairing of chromosomes carrying genes with which zeste interacts influences the expression of these genes. To explore the possible functions of the zeste gene product in this process, we have determined the DNA sequences both of a fragment of Drosophila genomic DNA capable of rescuing mutant zeste phenotypes, and of a near full-length cDNA clone derived from the 2.4-kb zeste mRNA. These data show that the zeste gene is interrupted by two small introns, and suggest that the majority of zeste sequences are contained within an intron of another transcriptional unit of opposite polarity. A large region of the predicted zeste product is comprised almost exclusively of glutamine and alanine residues. A domain near the N terminus of this protein, which is sufficient for site-specific DNA binding, is highly charged, as is the C-terminal region of the protein. A breakpoint of the rearrangement In (1)e(bx), which is associated with a za-like phenotype, is found within sequences encoding the zeste product, and would produce a truncated protein. The neomorphic mutation zv77h is correlated with a 300-bp deletion of sequences determining the untranslated 5' leader of the zeste messenger, but may also remove the initiating ATG codon, resulting in a zeste protein with an altered N terminus.