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The packaging of DNA into nucleosomal structures limits access for templated processes such as transcription and DNA repair. The repositioning or ejection of nucleosomes is therefore critically important for regulated events, including gene expression. This activity is provided by chromatin remodeling complexes, or remodelers, which are typically large, multisubunit complexes that use an ATPase subunit to translocate the DNA. Many remodelers contain pairs or multimers of actin-related proteins (ARPs) that contact the helicase-SANT-associated (HSA) domain within the catalytic ATPase subunit and are thought to regulate ATPase activity. Here, we determined the structure of a four-protein subcomplex within the SWI/SNF remodeler that comprises the Snf2 HSA domain, Arp7, Arp9, and repressor of Ty1 transposition, gene 102 (Rtt102). Surprisingly, unlike characterized actin-actin associations, the two ARPs pack like spoons and straddle the HSA domain, which forms a 92-Å-long helix. The ARP-HSA interactions are reminiscent of contacts between actin and many binding partners and are quite different from those in the Arp2/3 complex. Rtt102 wraps around one side of the complex in a highly extended conformation that contacts both ARPs and therefore stabilizes the complex, yet functions to reduce by ∼2.4-fold the remodeling and ATPase activity of complexes containing the Snf2 ATPase domain. Thus, our structure provides a foundation for developing models of remodeler function, including mechanisms of coupling between ARPs and the ATPase translocation activity.

We identify the helicase-SANT-associated (HSA) domain as the primary binding platform for nuclear actin-related proteins (ARPs) and actin. Individual HSA domains from chromatin remodelers (RSC, yeast SWI-SNF, human SWI-SNF, SWR1 and INO80) or modifiers (NuA4) reconstitute their respective ARP-ARP or ARP-actin modules. In RSC, the HSA domain resides on the catalytic ATPase subunit Sth1. The Sth1 HSA is essential in vivo, and its omission causes the specific loss of ARPs and a moderate reduction in ATPase activity. Genetic selections for arp suppressors yielded specific gain-of-function mutations in two new domains in Sth1, the post-HSA domain and protrusion 1, which are essential for RSC function in vivo but not ARP association. Taken together, we define the role of the HSA domain and provide evidence for a regulatory relationship involving the ARP-HSA module and two new functional domains conserved in remodeler ATPases that contain ARPs.

To establish a simple and an efficient system to minimize the environmental risk of genetically modified plants, we tested the applicability of the barnase/barstar system in conferring bisexual sterility; that is, in preventing plants setting seeds by self-fertilization and out-crossing. Transgenic tobacco plants were generated to express barnase, a cell death inducing ribonuclease, under the control of the gamete-specific AtDMC1 promoter, and barstar, a specific inhibitor of barnase, under the control of the ACT2 promoter, which is constitutively active in almost all tissues except gametes. In contrast to control plants harboring the barstar expression unit only, which set seeds normally with self-pollination, all transformants harboring both barnase and barstar were bisexually sterile. They produced aberrant anthers containing no detectable pollen and failed to set seeds even after pollination with wild-type tobacco pollen.

A novel male-sterile mutant of Arabidopsis thaliana was isolated by means of T-DNA tagging. Pollen abortion of the mutant was evident after microspore release, and pollen grains were completely absent at anthesis. Transmission electron microscope analysis revealed that primexine was coarsely developed, and that although sporopollenin was produced, it was not deposited onto the microspore plasma membrane. The sporopollenin that failed to be deposited aggregated and accumulated within the locule and on the locule wall. Finally, as no exine formation was observed, the mutant was named nef1. The plastoglobuli within the plastids of the tapetum were reduced, and lipid accumulation was considerably decreased. The mutant had a significantly altered leaf chloroplast ultrastructure and showed various growth defects. Lipid analysis revealed that the total lipid content in nef1 was lower than that in the wild type, which indicated that Nef1 was involved in lipid metabolism. Cloning of the full-length Nef1 indicated that the gene encodes a novel plant protein of 1123 amino acids with limited sequence similarities to membrane proteins or transporter-like proteins, and the NEF1 is predicted to be a plastid integral membrane protein. Motif analysis revealed that NEF1 contains prokaryotic membrane lipoprotein lipid attachment sites that are involved in maintaining cell envelope integrity. It is predicted that the Nef1 encodes a membrane protein that maintains the envelope integrity in the plastids.

A mutant exhibiting conditional male sterility, in which fertility was restored under conditions of high humidity, was identified in T-DNA tagged lines of Arabidopsis thaliana. Scanning electron microscopy (SEM) demonstrated that the pollen surface was almost smooth and the reticulate pattern not prominent. Thus, the mutant was named faceless pollen-1 (flp1). Transmission electron microscopy (TEM) revealed that the smooth appearance was due to tryphine filling in the exine cavities and covering the pollen surface. The lipid droplets in the tryphine of mutant pollen were smaller and more numerous than those of the wild type. SEM analysis also demonstrated that pollen exine was easily damaged by acetolysis, suggesting that a component of exine, sporopollenin, was defective in the mutant. In addition, the stems and siliques had reduced amounts of wax crystals. A predicted amino acid sequence of the cDNA that corresponded to the tagged gene, fip1, showed sequence similarity to proteins involved in wax biosynthesis. The FLP1 protein is likely to play a role in the synthesis of the components of tryphine, sporopollenin of exine and the wax of stems and siliques.

NF-kappa B regulates normal and pathological processes, including neoplasia, in a tissue-context-dependent manner. In skin, NF-kappa B is implicated in epidermal homeostasis as well as in the pathogenesis of squamous cell carcinoma; however, its function in the underlying mesenchymal dermis has been unclear. To gain insight into NF-kappa B roles in these two adjacent cutaneous tissue compartments, NF-kappa B effects on expression of 12 435 genes were determined in epidermal keratinocytes and dermal fibroblasts. Although NF-kappa B induced proinflammatory and antiapoptotic genes in both settings, it exhibited divergent effects on growth regulatory genes. In keratinocytes, but not in fibroblasts, NF-kappa B induced p21(CIP1), which was sufficient to inhibit growth of both cell types. Levels of growth inhibitory factor (GIF), in contrast, were increased by NF-kappa B in both settings but inhibited growth only in keratinocytes. These findings indicate that transcription factors such as NF-kappa B can program tissue-selective effects via both differential target gene induction as well as by inducing common targets that exert differing effects depending on cellular lineage.

Self-incompatibility in Brassica species is regulated by a set of S-locus genes: SLG, SRK, and SP11/SCR. In the vicinity of the S-locus genes, several expressed genes, SLL2 and SP2/ClpP, etc., were identified in B. campestris. Arabidopsis thaliana is a self-compatible Brassica relative, and its complete genome has been sequenced. From comparison of the genomic sequences between B. campestris and A. thaliana, microsynteny between gene clusters of Arabidopsis and Brassica SLL2 regions was observed, though the S-locus genes, SLG, SRK, and SP11/SCR were not found in the region of Arabidopsis. Almost all genes predicted in this region of Arabidopsis were expressed in both vegetative and reproductive organs, suggesting that the genes in the SLL2 region might not be related to self-incompatibility. Considering the recent speculation that the S-locus genes were translocated as a single unit between Arabidopsis and Brassica, the translocation might have occurred in the region between the SLL2 and SP7 genes.

Many flowering plants possess self-incompatibility (SI) systems to prevent inbreeding. SI in Brassica species is controlled by a single S locus with multiple alleles. In recent years, much progress has been made in determining the male and female S determinant in Brassica species. In the female, a gain-of-function experiment clearly demonstrated that SRK was the sole S determinant, and that SLG enhanced the SI recognition process. By contrast, the male S determinant (termed SP11/SCR) was identified in the course of genome analysis of S locus to be a small cysteine-rich protein, which was classified as a pollen coat protein. This SP11/SCR may function as a ligand for the S domain of SRK in the SI recognition reaction of Brassica species.

Self-incompatibility (SI) systems prevent self-pollination and promote outbreeding. In Brassica, the SI genes SLG (for S-locus glycoprotein) and SRK (for S-receptor kinase) are members of the S multigene family, which share the SLG-like domain (S domain), which encodes a putative receptor. We have cloned members of the S multigene family from the S9 haplotype of B. campestris (syn. rapa). In addition, eight distinct genomic regions harboring 10 SLG/SRK-like genes were characterized in the present study. Sequence analysis revealed two novel SRK-like genes, BcRK3 and BcRK6 (for B. campestris receptor kinases 3 and 6, respectively). Other genes that were characterized included SFR2 (for S gene family receptor 2), SLR2 (for S locus related gene 2), and a pseudogene. Based on phylogenetic analysis of the nucleotide sequences of the S domain regions, SLG and SRK appear to be distinct from other members of the S multigene family. Linkage analysis showed that most members of the S multigene family are dispersed in the Brassica genome, and that SLR1 (S locus related gene 1) is not linked to the SLR2 in B. campestris.

Many flowering plants have evolved self-incompatibility (SI) systems to prevent inbreeding. In the Brassicaceae, SI is genetically controlled by a single polymorphic locus, termed the S-locus. Pollen rejection occurs when stigma and pollen share the same S-haplotype. Recognition of S-haplotype specificity has recently been shown to involve at least two S-locus genes, S-receptor kinase (SRK) and S-locus protein 11 or S-locus Cys-rich (SP11/SCR). SRK encodes a polymorphic membrane-spanning protein kinase, which is the sole female determinant of the S-haplotype specificity. SP11/SCR encodes a highly polymorphic Cys-rich small basic protein specifically expressed in the anther tapetum and in pollen. In cauliflower (B. oleracea), the gain-of-function approach has demonstrated that an allele of SP11/SCR encodes the male determinant of S-specificity. Here we examined the function of two alleles of SP11/SCR of B. rapa by the same approach and further established that SP11/SCR is the sole male determinant of SI in the genus Brassica sp. Our results also suggested that the 522-bp 5'-upstream region of the S9-SP11 gene used to drive the transgene contained all the regulatory elements required for the unique sporophytic/gametophytic expression observed for the native SP11 gene. Promoter deletion analyses suggested that the highly conserved 192-bp upstream region was sufficient for driving this unique expression. Furthermore, immunohistochemical analyses revealed that the protein product of the SP11 transgene was present in the tapetum and pollen, and that in pollen of late developmental stages, the SP11 protein was mainly localized in the pollen coat, a finding consistent with its expected biological role.

Self-incompatibility (SI) in Brassica is sporophytically controlled by the multi-allelic S locus. SI phenotypes of the stigma and pollen in an S heterozygote are determined by the two S haplotypes it carries; the two haplotypes may be co-dominant or exhibit a dominant/recessive relationship. Because the S receptor kinase (SRK) gene of the S locus was recently shown to determine the S haplotype specificity of the stigma, we wished to investigate whether SRK also plays a role in the dominance relationships between S haplotypes. We crossed plants carrying an SRK28 transgene with plants homozygous for one of five S haplotypes that are either co-dominant with, or recessive to, S28 haplotype in the stigma, and analyzed the SI phenotypes of the progeny. In all cases, the SI phenotype of the stigma of plants carrying the SRK28 transgene could be predicted by the known dominance relationships between the S haplotype(s) and the S28 haplotype. Moreover, in the S43 homozygote carrying the SRK28 transgene where the S43 phenotype in the stigma was masked by the presence of the SRK28, the transcript level of SRK28 was found to be much lower than that of SRK43. All these results suggest that the dominance relationships between S haplotypes in the stigma are determined by SRK, but not by virtue of its relative expression level.

Nuclear factor kappaB (NF-kappaB) gene-regulatory proteins play important roles in inflammation, neoplasia, and programmed cell death. Recently, blockade of NF-kappaB function has been shown to result in epithelial hyperplasia, suggesting a potential role for NF-kappaB in negative growth regulation. We expressed active NF-kappaB subunits in normal epithelial cells and found that NF-kappaB profoundly inhibits cell cycle progression. This growth inhibition is resistant to mitogenic stimuli and is accompanied by other features of irreversible growth arrest. NF-kappaB-triggered cell cycle arrest is also associated with selective induction of the cyclin-dependent kinase inhibitor p21CiP1, with overexpression of p21(Cip1) alone inducing findings similar to those seen with NF-kappaB in vitro. An active NF-kappaB subunit expressed in the epidermis of p21(CiP1-/- mice, however, displays only partial growth-inhibitory effects, suggesting that full NF-kappaB growth inhibition is only partially p21(Cip1) dependent in this setting. These data indicate that NF-kappaB can trigger cell cycle arrest in epithelial cells in association with selective induction of a cell cycle inhibitor.

Self-incompatibility (SI) enables flowering plants to discriminate between self- and non-self-pollen. In Brassica, SI is controlled by the highly polymorphic S locus. The recently identified male determinant, termed SP11 or SCR, is thought to be the ligand of S receptor kinase, the female determinant. To examine functional and evolutionary properties of SP11, we cloned 14 alleles from class-I S haplotypes of Brassica campestris and carried out sequence analyses. The sequences of mature SP11 proteins are highly divergent, except for the presence of conserved cysteines. The phylogenetic trees suggest possible co-evolution of the genes encoding the male and female determinants.

Self-incompatible (SI) Brassica rapa (syn. B. campestris) was transformed with an antisense SLG gene by using SLG8 cDNA isolated from the B. campestris S8 homozygote. Two transformed lines were obtained and analyzed. Northern blot and Western blot analyses revealed that endogenous SLG and SRK were greatly reduced of the transcriptional and translational levels in the transformant. Pollination experiments confirmed that their SI phenotype had broken down. In addition, the progeny with the antisense SLG gene, resulting from self- or cross-pollination of the transgenic plant, also showed the self-compatible phenotype. The breakdown of SI in the tranformants was due to the change in property of the stigma and not of the pollen. These results provide strong evidence that SLG and/or SRK is implicated in the pollen-stigma recognition of SI and that they act only as stigmatic factors.

Adhesion of pollen grains to the stigmatic surface is a critical step during sexual reproduction in plants. In Brassica, S locus-related glycoprotein 1 (SLR1), a stigma-specific protein belonging to the S gene family of proteins, has been shown to be involved in this step. However, the identity of the interacting counterpart in pollen and the molecular mechanism of this interaction have not been determined. Using an optical biosensor immobilized with S gene family proteins, we detected strong SLR1-binding activity in pollen coat extracts of Brassica campestris. Two SLR1-binding proteins, named SLR1-BP1 and SLR1-BP2, were identified and purified by the combination of SLR1 affinity column chromatography and reverse-phase HPLC. Sequence analyses revealed that these two proteins (i) differ only in that a proline residue near the N terminus is hydroxylated in SLR1-BP1 but not in SLR1-BP2, and (ii) are members of the class A pollen coat protein (PCP) family, which includes PCP-A1, an SLG (S locus glycoprotein)-binding protein isolated from Brassica oleracea. Kinetic analysis showed that SLR1-BP1 and SLR1-BP2 specifically bound SLR1 with high affinity (K(d) = 5.6 and 4.4 nM, respectively). The SLR1-BP gene was specifically expressed in pollen at late stages of development, and its sequence is highly conserved in Brassica species with the A genome.

The self-incompatibility possessed by Brassica is an intraspecific reproductive barrier by which the stigma rejects self-pollen but accepts non-self-pollen for fertilization. The molecular/biochemical bases of recognition and rejection have been intensively studied. Self-incompatibility in Brassica is sporophytically controlled by the polymorphic S locus. Two tightly linked polymorphic genes at the S locus, S receptor kinase gene (SRK) and S locus glycoprotein gene (SLG), are specifically expressed in the papillar cells of the stigma, and analyses of self-compatible lines of Brassica have suggested that together they control stigma function in self-incompatibility interactions. Here we show, by transforming self-incompatible plants of Brassica rapa with an SRK28 and an SLG28 transgene separately, that expression of SRK28 alone, but not SLG28 alone, conferred the ability to reject self (S28)-pollen on the transgenic plants. We also show that the ability of SRK28 to reject S28 pollen was enhanced by SLG28. We conclude that SRK alone determines S haplotype specificity of the stigma, and that SLG acts to promote a full manifestation of the self-incompatibility response.

Many flowering plants possess self-incompatibility (SI) systems that prevent inbreeding. In Brassica, SI is controlled by a single polymorphic locus, the S locus. Two highly polymorphic S locus genes, SLG (S locus glycoprotein) and SRK (S receptor kinase), have been identified, both of which are expressed predominantly in the stigmatic papillar cell. We have shown recently that SRK is the determinant of the S haplotype specificity of the stigma. SRK is thought to serve as a receptor for a pollen ligand, which presumably is encoded by another polymorphic gene at the S locus. We previously have identified an S locus gene, SP11 (S locus protein 11), of the S(9) haplotype of Brassica campestris and proposed that it potentially encodes the pollen ligand. SP11 is a novel member of the PCP (pollen coat protein) family of proteins, some members of which have been shown to interact with SLG. In this work, we identified the SP11 gene from three additional S haplotypes and further characterized the gene. We found that (i) SP11 showed an S haplotype-specific sequence polymorphism; (ii) SP11 was located in the immediate flanking region of the SRK gene of the four S haplotypes examined; (iii) SP11 was expressed in the tapetum of the anther, a site consistent with sporophytic control of Brassica SI; and (iv) recombinant SP11 of the S(9) haplotype applied to papillar cells of S(9) stigmas, but not of S(8) stigmas, elicited SI response, resulting in inhibition of hydration of cross-pollen. All these results taken together strongly suggest that SP11 is the pollen S determinant in SI.

Specialized forms of physiologic cell death lacking certain characteristic morphologic features of apoptosis occur in terminally differentiating tissues, such as in the outer cell layers of epidermis. In these cell layers, NF-kappaB translocates from the cytoplasm to the nucleus and induces target gene expression. In light of its potent role in regulating apoptotic cell death in other tissues, NF-kappaB activation in these cells suggests that this transcription factor regulates cell death during terminal differentiation. Here, we show that NF-kappaB protects normal epithelial cells from apoptosis induced by both TNFalpha and Fas, whereas NF-kappaB blockade enhances susceptibility to death via both pathways. Expression of IkappaBalphaM under control of keratin promoter in transgenic mice caused a blockade of NF-kappaB function in the epidermis and provoked premature spontaneous cell death with apoptotic features. In normal tissue, expression of the known NF-kappaB-regulated antiapoptotic factors, TRAF1, TRAF2, c-IAP1, and c-IAP2, is most pronounced in outer epidermis. In transgenic mice, NF-kappaB blockade suppressed this expression, whereas NF-kappaB activation augmented it, consistent with regulation of cell death by these NF-kappaB effector proteins. These data identify a new role for NF-kappaB in preventing premature apoptosis in cells committed to undergoing physiologic cell death and indicate that, in stratified epithelium, such cell death normally proceeds via a distinct pathway that is resistant to NF-kappaB and its antiapoptotic target effector genes.

In Brassica, two self-incompatibility genes, encoding SLG (S locus glycoprotein) and SRK (S-receptor kinase), are located at the S locus and expressed in the stigma. Recent molecular analysis has revealed that the S locus is highly polymorphic and contains several genes, i.e., SLG, SRK, the as-yet-unidentified pollen S gene(s), and other linked genes. In the present study, we searched for expressed sequences in a 76-kb SLG/SRK region of the S(9) haplotype of Brassica campestris (syn. rapa) and identified 10 genes in addition to the four previously identified (SLG(9), SRK(9), SAE1, and SLL2) in this haplotype. This gene density (1 gene/5.4 kb) suggests that the S locus is embedded in a gene-rich region of the genome. The average G + C content in this region is 32.6%. An En/Spm-type transposon-like element was found downstream of SLG(9). Among the genes we identified that had not previously been found to be linked to the S locus were genes encoding a small cysteine-rich protein, a J-domain protein, and an antisilencing protein (ASF1) homologue. The small cysteine-rich protein was similar to a pollen coat protein, named PCP-A1, which had previously been shown to bind SLG.

Self-incompatibility (SI) in Brassicaceae is genetically controlled by the S locus complex in which S locus glycoprotein (SLG) and S receptor kinase (SRK) genes have been identified, and these two genes encoding stigma proteins are believed to play important roles in SI recognition reaction. Here we introduced the SLG43 gene of Brassica rapa into a self-incompatible cultivar, Osome, of B. rapa, and examined the effect of this transgene on the SI behavior of the transgenic plants. Preliminary pollination experiments demonstrated that Osome carried S52 and S60, and both were codominant in stigma, but S52 was dominant to S60 in pollen. S43 was found to be recessive to S52 and codominant with S60 in stigma. The nucleotide sequence of SLG43 was more similar to that of SLG52 (87.8% identity) than to that of SLG60 (74.8% identity). Three of the ten primary transformants (designated No. 1 to No. 10) were either completely (No. 9) or partially (No. 6 and No. 7) self-compatible; the SI phenotype of the stigma was changed from S52S60 to S60, but the SI phenotype of the pollen was not altered. In these three plants, the mRNA and protein levels of both SLG43 and SLG52 were reduced, whereas those of SLG60 were not. All the plants in the selfed progeny of No. 9 and No. 6 regained SI and they produced a normal level of SLG52. These results suggest that the alteration of the SI phenotype of the stigma in the transformants Nos. 6, 7, and 9 was the result of specific co-suppression between the SLG43 transgene and the endogenous SLG52 gene. Three of the transformants (Nos. 5, 8 and 10) produced SLG43 protein, but their SI phenotype was not altered. The S60 homozygotes in the selfed progeny of No. 10 which produced the highest level of SLG43 were studied because S43 was codominant with S60 in the stigma. They produced SLG43 at approximately the same level as did S43S60 heterozygotes, but did not show S43 haplotype specificity at the stigma side. We conclude that SLG is necessary for the expression of the S haplotype specificity in the stigma but the introduction of SLG alone is not sufficient for conferring a novel S haplotype specificity to the stigma.

Early in infection of Bacillus subtilis by bacteriophage SPO1, the synthesis of most host-specific macromolecules is replaced by the corresponding phage-specific biosyntheses. It is believed that this subversion of the host biosynthetic machinery is accomplished primarily by a cluster of early genes in the SPO1 terminal redundancy. Here we analyze the nucleotide sequence of this 11.5-kb "host-takeover module," which appears to be designed for particularly efficient expression. Promoters, ribosome-binding sites, and codon usage statistics all show characteristics known to be associated with efficient function in B. subtilis. The promoters and ribosome-binding sites have additional conserved features which are not characteristic of their host counterparts and which may be important for competition with host genes for the cellular biosynthetic machinery. The module includes 24 genes, tightly packed into 12 operons driven by the previously identified early promoters PE1 to PE12. The genes are smaller than average, with half of them having fewer than 100 codons. Most of their inferred products show little similarity to known proteins, although zinc finger, trans-membrane, and RNA polymerase-binding domains were identified. Transcription-termination and RNase III cleavage sites were found at appropriate locations.

In Brassica species that exhibit self-incompatibility, two genes, SLG and SRK, at the S locus are involved in the recognition reaction with self and non-self pollen. From a pollen-recessive S29 haplotype of Brassica rapa, both cDNA and genomic DNA clones for these two genes were isolated and characterized. The nucleotide sequence for the S domain of SRK29 showed a high degree of similarity with that of SLG29, and they belong to Class II type. RNA gel blot analysis showed that the transcript of SLG29 consisted of the first and second exons, and no other transcript containing any part of the intron sequence was detected. Because no transmembrane domain was encoded by the second exon of SLG29, SLG29 was designated a secreted type glycoprotein. SLGs of two other pollen-recessive haplotypes, S40 and S44, of B. rapa also had a similar structure to that of SLG29. Previously, SLG2 from a pollen-recessive haplotype, S2, of Brassica oleracea was found to produce two different transcripts, one for the secreted type glycoprotein and the other for a putative membrane-anchored form of SLG. Therefore, the nature of these SLGs from pollen-recessive haplotypes of B. rapa is different from that of SLG2 of B. oleracea.

pim1-d1ts was previously identified in a visual screen for fission yeast mutants unable to complete the mitosis-to-interphase transition. pim1+ encodes the guanine nucleotide exchange factor (GEF) for the spi1 GTPase. Perturbations of this GTPase system by either mutation or overproduction of its regulatory proteins cause cells to arrest with postmitotic condensed chromosomes, an unreplicated genome, and a wide medial septum. The septation phenotype of pim1-d1ts was used as the basis for a more extensive screen for this novel class of sns (septated, not in S-phase) mutants. Seventeen mutants representing 14 complementation groups were isolated. Three strains, sns-A3, sns-A5, and sns-A6, representing two different alleles, are mutated in the pim1+ gene. Of the 13 non-pim1ts sns complementation groups, 11 showed genetic interactions with the spi1 GTPase system. The genes mutated in 10 sns strains were synthetically lethal with pim1-d1, and six sns strains were hypersensitive to overexpression of one or more of the known components of the spil GTPase system. Epistasis analysis places the action of the genes mutated in nine of these strains downstream of pim1+ and the action of one gene upstream of pim1+. Three strains, sns-A2, sns-B1, and sns-B9, showed genetic interaction with the spil GTPase system in every test performed. sns-B1 and sns-B9 are likely to identify downstream targets, whereas sns-A2 is likely to identify upstream regulators of the spi1 GTPase system that are required for the mitosis-to-interphase transition.