No abstract

Chemokine receptors (CCR5, CXCR4 and CCR2) have been shown to be important co-receptors for HIV infection. Mutations at CCR5 (CCR5-∆32), CCR2 (CCR2-64I), and stromal-derived factor SDF1 (SDF1-3'A), a primary ligand for CXCR4, are known to have protective effects against HIV-1 infection and the onset of AIDS symptoms. We studied the three-locus genotype frequency distributions in 70 worldwide populations from a sample of 2341 individuals without any known history of HIV-1 infection and AIDS symptoms. From these data, we estimated the risk of AIDS onset (relative hazard, RH) of each population. This survey shows that the substantial allele frequency differences of each of these mutations translate into an extensive variation in relative hazards for AIDS in worldwide populations. However, no evidence of natural selection against the mutant gene carriers is detected. Finally, the combined three-locus genotype data predict the highest relative hazard (RH) in South-East Asia and Africa where AIDS is known to be more prevalent. European Journal of Human Genetics (2000) 8, 975-979.

Rhythmic activation and repression of clock gene transcription is essential for the functions of eukaryotic circadian clocks. In the Neurospora circadian oscillator, frequency (frq) transcription requires the WHITE COLLAR (WC) complex. Here, we show that the transcriptional corepressor regulation of conidiation-1 (RCO-1) is essential for clock function by regulating frq transcription. In rco-1 mutants, both overt and molecular rhythms are abolished, frq mRNA levels are constantly high, and WC binding to the frq promoter is dramatically reduced. Surprisingly, frq mRNA levels were constantly high in the rco-1 wc double mutants, indicating that RCO-1 suppresses WC-independent transcription and promotes WC complex binding to the frq promoter. Furthermore, RCO-1 is required for maintaining normal chromatin structure at the frq locus. Deletion of H3K36 methyltransferase su(var)3-9-enhancerof-zeste-trithorax-2 (SET-2) or the chromatin remodeling factor CHD-1 leads to WC-independent frq transcription and loss of overt rhythms. Together, our results uncover a previously unexpected regulatory mechanism for clock gene transcription. E ukaryotic circadian oscillators are based on autoregulatory negative feedback loops, in which positive and negative elements drive rhythmic gene expression (1-4). Despite the evolutionary distance, the circadian oscillator mechanism of the filamentous fungus Neurospora crassa is very similar to the mechanisms of animals (5-7). In Neurospora, Drosophila, and mammals, the positive elements of the circadian negative feedback loops are heterodimeric transcriptional activation complexes formed by a pair of Per-Arnt-Sim (PAS) domaincontaining transcription factors that rhythmically activate the transcription of the negative elements.In the core Neurospora circadian negative feedback loop, the positive element is the heterodimeric WHITE COLLAR (WC) complex of WC-1 and WC-2 (8-15). The WC complex binds to the promoter elements of the frequency ( frq) gene and activates its transcription. To close the negative feedback loop, FRQ and its partner FRQ-interacting RNA helicase (FRH) form a complex that inhibits transcription of frq by mediating FRQ/FRH complex-dependent WC phosphorylation, a process that inhibits WC complex activity and promotes its cytoplasmic localization (13,14,(16)(17)(18)(19)(20)(21). FRQ is progressively phosphorylated by several kinases and degraded through the ubiquitin proteasome pathway (13,(22)(23)(24). After the degradation of FRQ protein, the WC complex is reactivated, and a new cycle begins. The rhythmic activation and repression of frq transcription generate rhythmic frq mRNA levels, which are the major basis of circadian gene expression.WC-1 and WC-2 are the only previously known transcription factors required for the frq transcription. In wc-1 or wc-2 mutants, the levels of frq mRNA and FRQ protein are extremely low (11), leading to the conclusion that WC complex is the major-and perhaps, the only-regulator of frq transcription. There are two WC complex binding sites on f...

The circadian system in Neurospora is based on the transcriptional/translational feedback loops and rhythmic frequency (frq) transcription requires the WHITE COLLAR (WC) complex. Our previous paper has shown that frq could be transcribed in a WC-independent pathway in a strain lacking the histone H3K36 methyltransferase, SET-2 (su(var)3-9-enhancer-of-zestetrithorax-2) (1), but the mechanism was unclear. Here we disclose that loss of histone H3K36 methylation, due to either deletion of SET-2 or H3K36R mutation, results in arrhythmic frq transcription and loss of overt rhythmicity. Histone acetylation at frq locus increases in set-2 KO mutant. Consistent with these results, loss of H3K36 methylation readers, histone deacetylase RPD-3 (reduced potassium dependence 3) or EAF-3 (essential SAS-related acetyltransferase-associated factor 3), also leads to hyperacetylation of histone at frq locus and WC-independent frq expression, suggesting that proper chromatin modification at frq locus is required for circadian clock operation. Furthermore, a mutant strain with three amino acid substitutions (histone H3 lysine 9, 14, and 18 to glutamine) was generated to mimic the strain with hyperacetylation state of histone H3. H3K9QK14QK18Q mutant exhibits the same defective clock phenotype as rpd-3 KO mutant. Our results support a scenario in which H3K36 methylation is required to establish a permissive chromatin state for circadian frq transcription by maintaining proper acetylation status at frq locus.Despite evolutionary distance, circadian clock oscillators are conserved among the organisms to perform their cellular and behavioral activities. The eukaryotic circadian clock oscillator contains positive and negative elements that form auto-regulatory negative feedback loops (2-8).In the Neurospora circadian negative feedback loop, the GATA-family transcription factors WHITE COLLAR-1 (WC-1) 3 and WHITE COLLAR-2 (WC-2) serve as positive elements. Typically, WC-1 and WC-2 form a heterodimer through their Per-Arnt-Sim (PAS) domain and rhythmically bind the promoter of the clock gene frequency (frq) to activate its transcription. FREQUENCY (FRQ) and its partner FRQ-interacting RNA helicase (FRH) act as negative elements. FRQ is the core factor in Neurospora oscillator, which determines the normal clock rhythm (9 -16). The translated FRQ interacts with FRH and forms the FFC (FRQ-FRH complex) to inhibit its own transcription by suppressing the activity of the WC complex. FRQ is progressively phosphorylated and degraded through the ubiquitin-proteasome pathway. Degradation of FRQ derepresses the WC complex, initiating a new circle of frq transcription (14,(17)(18)(19). The circadian oscillator creates a robust rhythmic system with a period of ϳ22 h in Neurospora crassa (17,20). Rhythmic activation and repression of frq transcription result in cyclic changes of frq mRNA abundance. In Neurospora, histone modifiers and chromatin remodelers are required for rhythmic transcription of frq gene and clock-controlled genes (1, 9, 22-24). Howeve...

Background: Quelling and DNA damage induce small RNA from repetitive DNA regions. Results: Genetic Screen identified RTT109 as a critical component of the siRNA production pathway in Neurospora. Conclusion: RTT109 acts as a histone H3K56 acetyltransferase to mediate siRNA biogenesis through its role in homologous recombination. Significance: This study identifies a new link between DNA damage response and small RNA production at the chromatin level.