No abstract

Xbp1, a transcriptional repressor of Saccharomyces cerevisiae with homology to Swi4 and Mbp1, is induced by stress and starvation during the mitotic cycle. It is also induced late in the meiotic cycle. Using RNA differential display, we find that genes encoding three cyclins (CLN1, CLN3, and CLB2), CYS3, and SMF2 are downregulated when Xbp1 is overexpressed and that Xbp1 can bind to sequences in their promoters. During meiosis, XBP1 is highly induced and its mRNA appears at the same time as DIT1 mRNA, but its expression remains high for up to 24 h. As such, it represents a new class of meiosis-specific genes. Xbp1-deficient cells are capable of forming viable gametes, although ascus formation is delayed by several hours. Furthermore, Xbp1 target genes are normally repressed late in meiosis, and loss of XBP1 results in their derepression. Interestingly, we find that a deletion of CLN1 also reduces the efficiency of sporulation and delays the meiotic program but that sporulation in a ⌬cln1 ⌬xbp1 strain is not further delayed. Thus, CLN1 may be Xbp1's primary target in meiotic cells. We hypothesize that CLN1 plays a role early in the meiotic program but must be repressed, by Xbp1, at later stages to promote efficient sporulation.

The transcription factor Mcm1 is regulated by adjacent binding of a variety of different factors regulating the expression of cell-type-specific, cell cycle-specific, and metabolic genes. In this work, we investigate a new class of Mcm1-regulated promoters that are cell cycle regulated and peak in late M-early G 1 phase of the cell cycle via a promoter element referred to as an early cell cycle box (ECB). Gel filtration experiments indicate that the ECB-specific DNA binding complex is over 200 kDa in size and includes Mcm1 and at least one additional protein. Using DNase I footprinting in vitro, we have observed protection of the ECB elements from the CLN3, SWI4, CDC6, and CDC47 promoters, which includes protection of the 16-bp palindrome to which Mcm1 dimers are known to bind as well as protection of extended flanking sequences. These flanking sequences influence the stability and the variety of complexes that form on the ECB elements, and base substitutions in the protected flank affect transcriptional activity of the element. Chromatin immunoprecipitations show that Mcm1 binds in vivo to ECB elements throughout the cell cycle and that binding is sensitive to carbon source changes.Mcm1, its mammalian counterpart (serum response factor), and other members of the MADS box family of transcription factors share a 56-amino-acid MADS box, carrying a conserved DNA binding and dimerization domain (42). They are combinatorial transcription factors in that they typically bind adjacent to and derive their regulatory specificity from other DNA binding and/or accessory factors (3,20,42). Figure 1 depicts four classes of transcription complexes that include Mcm1. Depending upon the context, Mcm1 can be a component of an activating or a repressing complex, and these complexes can be regulated by specific events in the cell cycle or in response to internal or external cues. For example, repression of a-specific genes in ␣ haploid and a/␣ diploid cells involves cooperative binding of ␣2 and Mcm1 to adjacent binding sites (22,39). Then ␣2 recruits the Ssn6/Tup1 repressor complex to the site and prevents transcription (16). In contrast, ␣-specific genes are induced by Mcm1 and another ␣-specific transcription factor, ␣1 (43). In a cells, ␣2 is not produced, so the repressing complex cannot be formed. In that case, Mcm1 cooperates with another haploid-specific transcription factor, Ste12, to activate transcription and confer pheromone responsiveness to these promoters (12,14).Mcm1 is also involved in the regulation of arginine metabolism (not shown), where it forms a complex with Arg82 and two other DNA binding proteins: Arg80 and -81 (13, 33). The presence of arginine is sensed by Arg82 and results in the activation of transcription of catabolic enzymes and repression of anabolic enzymes, depending upon the context. Arg82 has recently been identified as an inositol-1,4,5-triphosphate kinase, and this discovery suggests a direct connection between the lipid signaling cascade and gene regulation in yeast (35).Mcm1 also plays a rol...

The Cln3-Cdc28 kinase is required to activate the Swi4-Swi6 transcription complex which induces CLN1 and CLN2 transcription in late G 1 and drives the transition to S. Cln3 and Swi4 are both rate limiting for G 1 progression, and they are coordinately transcribed to peak at the M/G 1 boundary. Early cell cycle box (ECB) elements, which confer M/G 1 -specific transcription, have been found in both promoters, and elimination of all ECB elements from the CLN3 promoter causes both a loss of periodicity and Cln3-deficient phenotypes, which include an extended G 1 interval and increased cell volume. Mutants lacking the ECB elements in both the CLN3 and SWI4 promoters have low and deregulated levels of CLN transcripts, and the G 1 -to-S transition for these mutants is delayed and highly variable. These observations support the view that the coordinated rise of Cln3 and Swi4 levels mediated by ECB-dependent transcription controls the timing of the G 1 -to-S phase transition.

A genomic clone containing a novel closely linked human histone H2A/H2B gene pair has been isolated and sequenced along with extensive 5' and 3' flanking regions. Both genes are devoid of introns and code for core histone proteins. The nucleotide sequences are 84% and 87% homologous to the coding regions of a human genomic H2A and H2B gene, respectively. A comparison of the nucleotide-derived amino acid sequences shows that the histone H2A protein corresponds to the human H2A.1 subtype, whereas the H2B histone gene predicts an H2B protein sequence which is almost identical to the histone H2B.2 variant from human and bovine obtained by direct protein sequencing. The 3' flanking regions contain previously identified conserved sequence elements thought to be involved in transcription termination and processing of replication-dependent histone gene poly(A)- mRNAs. Primer extension analyses of the histone mRNAs encoded within this clone demonstrate that both genes are divergently transcribed from a 313 bp intergene promoter region. The spatial arrangement and orientation of two TATA-boxes, four CAAT-boxes, and one H2B-box within this region suggests that the linked genes share common promoter elements for transcriptional regulation.