BCOR 103 1st Edition Lecture 22 Outline of Last Lecture I Nuclear Structure and Function II Components III Transport Outline of Current Lecture I II III Nuclear Transport Regulation and RNA Open Mitosis Current Lecture Regulation of Nuclear Transport o Transcription factor import in response to cell signaling o NLS of NF kB is masked by IkB o Phosphorylation of IkB targets for degradation o Import of NF kB Dephosphorylation of Pho4 unmasks an NLS Selective import may be regulated in response to cell signaling Cell Signaling directs the import of transcription factor NF kB through the action of a kinase that phosphorylates the NF kB NLS is accessible to importin Nuclear import of the yeast transcription factor Pho4 is also regulated by phosphorylation Cell signaling activates a phosphatase that dephosphorylates Pho4 thereby allowing importin to access its NLS Bottom Line regulation of access of specific transcription factors to the nucleus acts to regulate the expression of the genes with which those transcription factors interact Androgen signaling regulation of nuclear import in response to a steroid hormone Nucleolus dynamic structure o Not membrane enclosed o Site of RNA Polymerase I RNAPI transcription of ribosomal RNA gene repeats o RNAPI transcription is essential for nucleolus self assembly o Co transcriptional assembly of rRNA with ribosomal proteins o Metabolically active human cells 5 10 million ribosomes cell Humans 200 rRNA gene tandem repeats on 5 chromosomes rRNA gene repeat encodes three different rRNAs 18S 5 8S and 28S These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute o The 4th rRNA 5S rRNA is not encoded within the tandem repeats but rather from 2 000 gene copies located within a single tandem array on human chromosome 1 outside the nucleolus and transcribed by RNAPIII Synthesis of mature 18S 5 8S and 28S rRNA requires both endonucleolytic and exonucleolytic processing of a pre rRNA Cleavage of the primary transcript occurs cotranscriptionally 18S rRNA is incorporated into the 40S ribosomal subunit 28S 5 8S and 5S are incorporated into the 60S ribosomal subunit Key Concept nucleolus is a self organizing structure o Self organization is dependent upon rRNA transcription by RNAPI o Blockage of rRNA transcription by DRB or actinomyosin D results in the disruption of the structure of the nucleolus Key Concept nucleolus is a highly dynamic structure whose activity is tightly coupled to the demand for protein synthesis Initial ribosomal subunit assembly occurs co transcriptionally within the nucleolus Despite the amorphous appearance of the interphase nucleus in the electron microscope the nucleus is highly organized This organization is essential for function of the nucleus especially gene expression Nuclear organization is dynamic FISH fluorescence in situ hybridization Chromosome Territories preferential positions of chromosomal DNA within the nucleusdynamic cell type specific Example Hemoglobin o Hemoglobin is a heterotetramer composed of alpha globin gene encoded on chromosome 6 and beta globin gene encoded on chromosome 11 proteins o The alpha and beta globin genes become spatially juxtaposed within the nucleus of hematopoietic cells upon transcriptional activation Key Concept dynamic changes in the positions of specific chromatin regions within the nucleus in response to cell development differentiation or cell signaling Nuclear sub components o Not membrane bound o Self organizing o Dynamic responsive to cell signaling o Functional significance of many of these compartments has yet to be determined Open mitosis dissolution of the nuclear envelope and consequent mixing of nuclear and cytoplasmic components Key concept sorting of nuclear and cytoplasmic components following cytokinesis is accomplished by nuclear import into the reassembling nucleus