RNA Polymerase II Activity and Nuclear Actin: Possible Roles of Nuclear Tropomyosin, Troponin and Ca²⁺ in Transcription in Striated Muscle Myocyte Nuclei

RNA polymerases are macromolecular machines that catalyze the synthesis of RNA macromolecules, the sequences of which is coded for by the sequences of regions of DNA macromolecules in the nucleus of eukaryotic cells, or nuclei in the case of many mature striated muscle cells, or myocytes, which are in many cases polynucleated. Herein, we review the evidence that transcription, the activity of RNA polymerases that is an essential step in gene expression, and processes related to maturation of eukaryotic RNA can be influenced by the macromolecule actin and its macromolecular complex of filamentous actin and its association with actin-binding proteins in the nucleus. We furthermore hypothesize that the macromolecular complexes of troponin and tropomyosin, which bind actin filaments in the cytoplasm of striated muscle myocytes to form thin filaments and which are also found in the nuclei of striated muscle myocytes and some cancerous cells, could modulate that influence of nuclear actin on transcription when present in a nucleus. Interestingly, troponin and tropomyosin could confer Ca2+-dependence to transcriptional modulation by nuclear actin, a mechanism that would complement Ca2+-dependent modulation of posttranslational modifications that influence gene expression.