Δυναμικά χαρακτηριστικά της μεταγραφής: η λειτουργική σημασία της ιστόνης Htz1 στην έναρξη της μεταγραφής την ενδοπυρινική τοποθέτηση και τη μεταγραφική μνήμη του συστήματων των γονιδίων GAL στο Saccharomyces cerevisiae

During the present study the significance of the histone variant Htz1 presence was studied at the yeast Saccharomyces cerevisiae model gene GAL1. It was established that the Htz1 nucleosomes are important for the recruitment of the Mediator complex during transcription initiation, providing an explanation of the positive role of this histone during transcription. Unexpectedly, the incorporation of the variant in a specific promoter nucleosome was found to depend on the corepressor Tup1-Ssn6. This finding suggests that gene-specific regulators might control the incorporation of Htz1 throughout promoters. Furthermore, it was established that this deposition takes place after the formation of canonical nucleosomes during promoter inactivation, regulated in part by Ssn6-Tup1. It is proposed that in this way the corepressor secures a rapid second activation. The fact that this incorporation was suggested by others to regulate the attachment of the GAL genes at the nuclear periphery and thus the phenomenon of transcriptional memory of the GAL genes, led to the examination of the role of Htz1 on both of these processes. It is concluded that Htz1 is not required for the retention at the periphery. Moreover, the enhanced affinity of the GAL genes for the nuclear pores was seen to be unnecessary for optimal transcription. On the other hand, this recruitment to the periphery, while taking place in a major population of nuclei, proved to be independent of transcription itself. Thus, it is proposed that this phenomenon is an escorting event but is not regulated by nor regulates transcription. Finally, histone variant Ζtz1 is not the determining factor of the transcriptional memory phenomenon. Employing a yeast heterocaryon assay is demonstrated that this phenomenon has no nuclear basis and Htz1 is not the determining factor. The cytoplasmic basis of transcriptional memory was found to depend on the yeast galactokinase enzyme Gal1, supporting its previously well described activating role