Programming the expression of the genome is essential for cell growth, differentiation, and development. The precise induction of defined sets of genes at specific stages is particularly important for cellular differentiation in eukaryotes.
Meiosis and spore morphogenesis in Saccharomyces cerevisiae
are developmental processes of this organism. Recent microarray experiments indicated that more than 1,000 genes are induced above background levels during these processes (8
). The initiation of the meiotic pathway is governed by a genetic signal, indicating that the cell is diploid, and a nutritional signal, indicating that the cell is being starved by the absence of both a fermentable carbon source and nitrogen. These signals induce the expression and activation of Ime1, which serves as the master switch for meiosis (for review, see references 12
, and 36
). Ime1 is a transcriptional activator of early meiosis-specific genes (EMGs). Of such genes IME2
, which encodes a serine/threonine protein kinase, has particular importance; because it, together with Ime1, participates in the normal activation of EMGs (for a review, see reference 18
A large number of EMG promoters, including that of IME2
, contain a 9-bp site called the upstream repressor sequence (URS1), which is constitutively bound by a zinc finger protein, Ume6. When the cells are under conditions for vegetative growth, with either glucose or acetate as the sole carbon source, Ume6 interacts with Rpd3-Sin3 histone deacetylase (HDAC)- and Isw2 chromatin-remodeling complexes to repress transcription (9
). The Isw2 complex promotes the formation of a nuclease-inaccessible chromatin structure upstream of the URS1 sequence at target genes by changing nucleosome positions, and the Rpd3-Sin3 complex deacetylates histones incorporating the URS1 site to enhance the repressed state (9
). Upon activation of EMGs, Ume6 functions as an activator by tethering Ime1 to URS1 (28
). This interaction between Ume6 and Ime1 requires the phosphorylation of Ime1 by Rim11 and potentially other kinases such as Mck1 (38
). For the efficient activation of EMGs, Gcn5 histone acetyltransferase (HAT) and the RSC chromatin remodeling complex play pivotal roles (5
); and the Set3 complex, which contains a putative histone methyltransferase and two HDACs, also affects the regulation of EMGs (20
). These studies indicate that the conversion of Ume6 from a repressor to an activator of EMG expression through the alteration of interacting partners and the regulation of the chromatin structure around the URS1 site by multiple chromatin remodelers are critical for the induction of EMGs and for the progression of meiosis. However, the molecular mechanisms for regulating the EMG expression in the context of chromatin are poorly understood.
Here we monitored the time course of the activation of IME2, focusing on the chromatin structure of the promoter region of this gene and the interplay between the factors known to be involved in the regulation.