A cause of aging in yeast is the accumulation of circular species of ribosomal DNA (rDNA) arising from the 100-200 tandemly repeated copies in the genome. We show here that mutation of the FOB1 gene slows the generation of these circles and thus extends life span. Fob1p is known to create a unidirectional block to replication forks in the rDNA. We show that Fob1p is a nucleolar protein, suggesting a direct involvement in the replication fork block. We propose that this block can trigger aging by causing chromosomal breaks, the repair of which results in the generation of rDNA circles. These findings may provide a novel link between metabolic rate and aging in yeast and, perhaps, higher organisms.

Fungi must recognize plant-specific signals to initiate subsequent morphogenetic events such as filamentation that lead to infection. Here we show that the plant hormone indoleacetic acid (IAA) induces adhesion and filamentation of Saccharomyces cerevisiae. Genome expression profiling of cells treated with IAA identified Yap1, a fungal specific transcription factor, as a key mediator of this response. Strains lacking YAP1 (yap1-1) are hypersensitive to growth on IAA because they accumulate more IAA than can wild type. Members of a family of transporters the amino acid͞ auxin:proton symport permeases with homology to AUX1, a putative IAA transporter from plants, are up-regulated in the yap1-1 mutant. Deletion of any one of these transporters makes yap1-1 mutants more resistant to IAA by decreasing its uptake. The permease mutants are defective in IAA perception and filamentation. The ability of a fungus to perceive a plant hormone that causes it to differentiate into an invasive form has important implications for plant-pathogen interactions.

Treating Arabidopsis roots with exogenous auxin results in dramatic changes in cellular processes including de novo induction of lateral roots which later emerge through the overlying cells. Microarray experiments reveal approximately 80 genes that are substantially up-regulated in the root over the first 12 h following auxin treatment. We hypothesize that the observed increase in expression of pectate lyase family genes leads to degradation of the pectin-rich middle lamellae, allowing cells in the parent root to separate cleanly. Differences in the degree of pectin methylation in lateral and parent roots may explain why lateral roots are not degraded themselves.

In the ribosomal DNA of Saccharomyces cerevisiae, sequences in the nontranscribed spacer 3 of the 35S ribosomal RNA gene are important to the polar arrest of replication forks at a site called the replication fork barrier (RFB) and also to the cis-acting, mitotic hyperrecombination site called HOT1. We have found that the RFB and HOT1 activity share some but not all of their essential sequences. Many of the mutations that reduce HOT1 recombination also decrease or eliminate fork arrest at one of two closely spaced RFB sites, RFB1 and RFB2. A simple model for the juxtaposition of RFB and HOT1 sequences is that the breakage of strands in replication forks arrested at RFB stimulates recombination. Contrary to this model, we show here that HOT1-stimulated recombination does not require the arrest of forks at the RFB. Therefore, while HOT1 activity is independent of replication fork arrest, HOT1 and RFB require some common sequences, suggesting the existence of a common trans-acting factor(s).The ribosomal DNA (rDNA) locus in the yeast Saccharomyces cerevisiae consists of 9.1-kb tandem repeats with the 35S rRNA gene, the much smaller 5S rRNA gene, and two nontranscribed spacer (NTS) regions (see Fig. 1) (see references 29 and 22 for reviews of sequence elements in the NTS). NTS2, located between the 5Ј ends of the two genes, contains the promoter for the 35S rRNA gene, a weak origin of replication named the rDNA ARS, and sequences essential for the cisacting mitotic recombination hot spot HOT1. The 35S RNA polymerase I transcriptional enhancer lies in NTS1 near the 3Ј end of the 35S gene. NTS1 also contains sequences important for the polar arrest of replication forks (replication fork barrier [RFB]) and HOT1. The extent of sequence overlap and the interdependence of these two events in DNA metabolism are unknown.The rDNA RFB was first identified in S. cerevisiae, when high-resolution two-dimensional (2D) gel electrophoresis revealed two closely spaced sites where forks arrest (2), herein called RFB1 and RFB2. RFBs appear to be a highly conserved feature of rDNAs, with barriers being found at the 3Ј end of the rRNA genes in a number of other organisms (9,21,23,32,36,38). The yeast RFBs efficiently block replication forks traveling in the direction opposite to 35S transcription, together impeding ϳ90% of encountered forks (2). Fork arrest is not a consequence of transcription per se, since replication forks still arrest at the RFB in cells lacking functional RNA polymerase I (2). The RFB sequences are also not inherently difficult to replicate (2), and thus fork arrest is thought to result from the binding of proteins at the RFB sequences. A protein-mediated mechanism of fork arrest in the rDNA RFB has also been implicated in peas and Tetrahymena thermophila (24, 37) and reported to involve the transcription-terminating factor TTF-I in mice and humans (8,23).HOT1 sequences from the rDNA, when assayed at ectopic sites in the genome, stimulate mitotic homologous recombination between intra-and interchromosomal repeats (...