Eukaryotic DNA is packaged in chromatin, whose repeating subunit, the nucleosome, consists of an octamer of histone proteins wrapped by about 147bp of DNA. This packaging affects the accessibility of DNA and hence any process that occurs on DNA, such as replication, repair, and transcription. An early observation from genome-wide nucleosome mapping in yeast was that genes had a surprisingly characteristic structure, which has motivated studies to understand what determines this architecture. Both sequence and trans acting factors are known to influence chromatin packaging, but the relative contributions of cis and trans determinants of nucleosome positioning is debated. Here we present data using genetic approaches to examine the contributions of cis and trans acting factors on nucleosome positioning in budding yeast.

We developed the use of yeast artificial chromosomes to exploit quantitative differences in the chromatin structures of different yeast species. This allows us to place approximately 150kb of sequence from any species into the S.cerevisiae cellular environment and compare the nucleosome positions on this same sequence in different environments to discover what features are variant and hence regulated by trans acting factors. This method allowed us to conclusively show that the great preponderance of nucleosomes are positioned by trans acting factors. We observe the maintenance of nucleosome depletion over some promoter sequences, but partial fill-in of NDRs in some of the YAC v promoters indicates that even this feature is regulated to varying extents by trans acting factors.

We are able to extend our use of evolutionary divergence in order to search for specific trans regulators whose effects vary between the species. We find that a subset of transcription factors can compete with histones to help generate some NDRs, with clear effects documented in a cbf1 deletion mutant. In addition, we find that Chd1p acts as a potential “molecular ruler” involved in defining the nucleosome repeat length differences between S.cerevisiae and K.lactis. The mechanism of this measurement is unclear as the alteration in activity is partially attributable to the N-terminal portion of the protein, for which there is no structural data. Our observations of a specialized chromatin structure at de novo transcriptional units along with results from nucleosome mapping in the absence of active transcription indicate that transcription plays a role in engineering genic nucleosome architecture. This work strongly supports the role of trans acting factors in setting up a dynamic, regulated chromatin structure that allows for robustness and fine-tuning of gene expression.