Current Research

The role of the Spt10 HAT-activator in cell cycle regulation of the yeast histone genes

We have shown that nucleosomes on the CUP1 promoter are acetylated in response to induction by copper and that this targeted acetylation is dependent on Spt10p, a putative histone acetyltransferase (HAT) [3]. SPT10 was originally identified as one of a set of SPT genes, mutations in which suppress phenotypes associated with insertion of a yeast transposable element into promoters. SPT10 is not an essential gene, but the null allele is associated with very slow growth and global defects in gene regulation. Spt10p activates the histone genes, which it regulates in conjunction with Spt21p, the Hir co-repressor and the SWI/SNF complex. We and others originally proposed that Spt10p might be a co-activator recruited to promoters by activators. However, we have shown recently that Spt10p is in fact a sequence-specific DNA binding protein that recognises the histone UAS elements [(G/A)TTCCN6TTCNC] [6]. Spt10p appears to be the activator of the core histone genes, which has been sought after for many years. We found that it binds with high affinity and with extraordinary positive cooperativity to pairs of histone UAS elements. Since pairs of histone UAS elements are found only in the core histone promoters and nowhere else in the yeast genome, there are no other predicted sites for Spt10p binding. We have presented evidence that the effects of Spt10p on other genes are indirect, mediated through global defects in chromatin structure arising from a deficit of histones in spt10 cells [6].

We are making rapid progress in understanding the biological functions of Spt10p [7, 10]. Spt10p appears to be only the second example of a sequence-specific DNA binding domain fused to a HAT domain. However, Spt10p has not yet been shown to possess HAT activity in vitro. We are using a variety of approaches to identify the acetyltransferase activity of Spt10p. In addition, we are attempting to identify proteins which interact with Spt10p. Spt10p binds to the histone UAS elements and therefore should be classified as an activator rather than a co-activator, but it does not have a conventional activation domain. Usually activators recruit HAT enzymes as co-activators. In the case of Spt10p, we suggest that Spt10p recruits an activation domain. Our current aim is to place our observations in their biological context of S-phase regulated expression of the histone genes.