The Rad53 protein kinase of Saccharomyces cerevisiae is required for checkpoints that prevent cell division in cells with damaged or incompletely replicated DNA. The Rad9 protein was phosphorylated in response to DNA damage, and phosphorylated Rad9 interacted with the COOH-terminal forkhead homology-associated (FHA) domain of Rad53. Inactivation of this domain abolished DNA damage-dependent Rad53 phosphorylation, G2/M cell cycle phase arrest, and increase of RNR3 transcription but did not affect replication inhibition-dependent Rad53 phosphorylation. Thus, Rad53 integrates DNA damage signals by coupling with phosphorylated Rad9. The hitherto uncharacterized FHA domain appears to be a modular protein-binding domain.

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We report here a synthetic-lethal screen in Caenorhabditis elegans that overcomes a number of obstacles associated with the analysis of functionally redundant genes. Using this approach, we have identified mutations that synthetically interact with lin-35/Rb, a SynMuv gene and the sole member of the Rb/pocket protein family in C. elegans. Unlike the original SynMuv screens, our approach is completely nonbiased and can theoretically be applied to any situation in which a mutation fails to produce a detectable phenotype. From this screen we have identified fzr-1, a gene that synthetically interacts with lin-35 to produce global defects in cell proliferation control. fzr-1 encodes the C. elegans homolog of Cdh1/Hct1/FZR, a gene product shown in other systems to regulate the APC cyclosome. We have also uncovered genetic interactions between fzr-1 and a subset of class B SynMuv genes, and between lin-35 and the putative SCF regulator lin-23. We propose that lin-35, fzr-1, and lin-23 function redundantly to control cell cycle progression through the regulation of cyclin levels. Genetic redundancy is a common feature of all eukaryotes, ensuring both a high degree of regulatory control and protection against mutational assault. It is also an impediment to biologists seeking to determine gene function through forward and reverse genetic approaches. Mutations in functionally redundant genes may show no obvious phenotype on their own but, when present in specific combinations, can lead to severe defects (i.e., a synthetic phenotype). Therefore, the ability to assign functions to this class of genes based solely on the availability and analysis of single mutants may be difficult or impossible.Estimates from yeast suggest that ∼40% of all genes may fail to show even weak phenotypes when deleted (Smith et al. 1996;Winzeler et al. 1999), whereas a recent large-scale functional analysis of Caenorhabditis elegans genes using RNAi methods failed to detect phenotypes for >85% of the genes examined (Fraser et al. 2000). In addition, recent small-scale deletion studies as well as previous mutational analyses suggest that functional disruptions of most genes in C. elegans fail to produce obvious phenotypic effects (for review, see Hodgkin 2001). This apparent lack of an observable gene function may be the result of limitations at the level of detection as well as in the methodology used in the case of RNAi analysis. However, it is also likely a consequence of functional overlap conferred by either structurally related proteins or by molecularly distinct but functionally connected pathways. Evidence from yeast suggests that this latter explanation may, in fact, be the more common cause of genetic redundancy (Winzeler et al. 1999; for review, see Tautz 2000;Wagner 2000).Given the difficulties in detecting synthetic interactions, relatively few examples of such nonhomologous genetic redundancies have been well characterized in C. elegans Johnson et al. 1981;Ferguson and Horvitz 1989;Davies et al. 1999). The best-known case of functiona...

Molting is an essential developmental process for the majority of animal species on Earth. During the molting process, which is a specialized form of extracellular matrix (ECM) remodeling, the old apical ECM, or cuticle, is replaced with a new one. Many of the genes and pathways identified as important for molting in nematodes are highly conserved in vertebrates and include regulators and components of vesicular trafficking, steroid-hormone signaling, developmental timers, and hedgehog-like signaling. In this review, we discuss what is known about molting, with a focus on studies in . We also describe the key structural elements of the cuticle that must be released, newly synthesized, or remodeled for proper molting to occur.

Transgenic Caenorhabditis elegans animals have been engineered to express wild-type and singleamino acid variants of a long form of human /3-amyloid peptide (A/3 1-42). These animals express high levels (~3OOng of A/3/mg of total protein) of apparently fulllength peptide, as determined by quantitative immunoblot. Expression of wild-type Afi in these animals leads to rapid production of amyloid deposits reactive with Congo red and thioflavin S. This model system has been used to examine the effect of Leu 17Pro, Leu17Val, Ala30-Pro, Met35Cys, and Met35Leu substitutions on the in vivo production of amyloid deposits. We find that the Leu17 Pro and Met 35Cys substitutions completely block the formation of thioflavin S-reactive deposits, implicating these as key residues for in vivo amyloid formation. We have also constructed transgenic strains expressing a novel A/I variant, the single-chain dimer. Animals expressing high levels of this variant also fail to produce thioflavin S-reactive deposits. Key Words: Caenorhabditis elegans-Transgenic-/3-Amyloid peptide-/3-Amyloid peptide variant-Aggregation.