The right part of the hrp cluster of Pseudomonas syringae pv. phaseolicola contains two regulatory genes, the previously described hrpS gene and an adjacent locus, hrpR. In this study we determined the sequence of hrpR and analysed the functional organization of the two genes. HrpR and HrpS show high sequence similarities to each other and to other response regulators of the two-component regulatory system. This has recently also been described for the hrpRS system of the closely related pathogen Pseudomonas syringae pv. syringae. The results of our genetic analyses strongly indicate that hrpS expression is regulated by the hrpR gene product. DNA-protein binding studies and site-directed mutagenesis of the hrpR sequence provided further evidence that HrpR activates hrpS transcription by binding to an activator site. This HrpR binding site was mapped in a fragment which is located 378-609 nucleotides upstream of the hrpS transcription start site. The hrpS transcription start site maps 179 nucleotides upstream of the initiation codon ATG, as determined by primer extension analysis, and is preceded by a typical -12/-24 promoter motif.

A ca. 20-kilobase (kb) region (hrp) that controls the interaction of Pseudomonas syringae pv. phaseolicola with its host (pathogenicity) and nonhost plants (hypersensitive reaction) was previously cloned and partially characterized. In this study we defined the limits and determined the nucleotide sequence of a hrp locus (hrpS) The plant pathogen Pseudomonas syringae pv. phaseolicola causes the halo blight disease on bean, its natural host, and elicits the hypersensitive response (HR), a resistanceassociated reaction, on several nonhost plants. In previous studies, we described a group of genes that are required for pathogenicity as well as for the elicitation of HR by this bacterium (21). We have designated these genes hrp (phonetic "harp," for hypersensitive reaction and pathogenicity) and have established that in strain NPS3121, the majority of them are clustered in a 20-to 22-kilobase (kb) region, the hrp cluster. HR is an expression of plant resistance to heterologous pathogens, but is also expressed in resistant cultivars of host species that carry resistance genes that are functionally correspondent in a gene-for-gene sense (10) to avirulence genes (avr) present in the pathogen (39,40). Significantly, the expression of race-or cultivar-specific HR requires a fully functional hrp region, in addition to avirulence genes (20).The mechanism by which the hrp genes determine or control the plant-pathogen interactions is not known. To gain further insight into the subject, detailed structural and functional analysis of hrp genes has been undertaken. In this report we present the DNA sequence, complementation analysis, and in vitro expression of one hrp locus, which we designated hrpS. We show that this locus contains a large open reading frame (ORF) and that the predicted translation product of this ORF is a 302-amino-acid protein. This protein shows significant homology to the highly conserved central domain of several procaryotic regulatory proteins that are involved in the activation or repression of diverse operons in enteric and plant symbiotic bacteria in response to different * Corresponding author. environmental or cellular signals. We also show that hrpS is a regulatory locus whose function is necessary for the expression of another hrp gene during the host-pathogen interaction. The short length of this protein compared with the length of other members of the group corroborates previous predictions of functional autonomy of their highly conserved domains (7) and invites speculation about the evolution of regulatory mechanisms important to Pseudomonas pathogenesis on plants. MATERIALS AND METHODS Enzymes

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In this study we describe a novel gene, which was isolated in an attempt to search for specific plant resistance genes of Arabidopsis against isolates of the phytopathogenic bacterium Xanthomonas campestris pv. campestris. The gene was cloned by differential screening of a genomic library of the Xcc 750-resistant ecotype Col-0, using cDNA populations derived from ecotype Col-0 and the Xcc 750-susceptible ecotype Oy-0. The isolated gene, CXc750, is differentially expressed in ecotypes of Arabidopsis thaliana. In addition, although highly expressed in uninfected plants, gene expression increases in response to pathogen attack. CXc750 potentially codes for a small, basic protein of about 10 kDa. The predicted protein product contains a potential signal leader peptide at the amino-terminal end but no ER retention sequence and no further transmembrane domain. This indicates that the gene product is transported to other compartments or out of the cell. The possible function of CXc750 as a member of the plant defense response system is discussed.

During the formation of polytene chromosomes in salivary glands of Drosophila hydei, the genes for ribosomal RNA (rDNA) are underreplicated relative to the rest of the genome. We have measured the number of rRNA genes with and without intervening sequences (ivs+ and ivs- genes) in polytene chromosomes of different genotypes. In the group of genotypes having a large number of ivs- rRNA genes polytenization only occurs within the cluster of ivs- genes. In each of these genotypes rDNA polytenization reaches a constant level of 150 ivs- genes per two chromatid sets (2C); X/X constitutions having two nucleolus organizers (NOs) in the diploid set polytenize the same amount of rDNA as X/O constitutions. In the group of genotypes with small ivs- gene numbers, the rDNA region involved in polytenization is longer and has an average length of 1,700 kb per NO, which is constant in these genotypes. Polytenization of rDNA is extended into the cluster of ivs+ genes, in spite of the fact that these genes appear to be nonfunctional. The smaller the number of ivs- genes, the greater the number of ivs+ genes that are polytenized in the NO. In these genotypes, X/X females replicate twice as much rDNA as X/O males, suggesting that both NOs of the diploid set are polytenized. A comparison of the pattern of spacer length heterogeneity in hybrids between different stocks also demonstrates that both NOs are replicated during polytenization.