This review will attempt to cover the present information on the multiple forms of eukaryotic DNA-dependent RNA polymerases, both at the structural and functional level. Nuclear RNA polymerases constitute a group of three large multimeric enzymes, each with a different and complex subunit structure and distinct specificity. The review will include a detailed description of their molecular structure. The current approaches to elucidate subunit function via chemical modification, phosphorylation, enzyme reconstitution, immunological studies, and mutant analysis will be described. In vitro reconstituted systems are available for the accurate transcription of cloned genes coding for rRNA, tRNA, 5 SRNA, and mRNA. These systems will be described with special attention to the cellular factors required for specific transcription. A section on future prospects will address questions concerning the significance of the complex subunit structure of the nuclear enzymes; the organization and regulation of the gene coding for RNA polymerase subunits; the obtention of mutants affected at the level of factors, or RNA polymerases; the mechanism of template recognition by factors and RNA polymerase.

Eukaryotic RNA polymerases.

Our previous study demonstrated that a high level of nuclear triiodothyronine receptors (NT3R), which are identical to the hepatic NT3R, exists in neuronal nuclei of the cerebral cortex from an adult rat brain. To investigate whether thyroid hormone acts through binding to nuclear receptors, we measured RNA polymerase activities in isolated neuronal and glial nuclei of cerebral cortices prepared from three groups of rats with different T3 levels: T3 (20 micrograms/100 g BW/d, for three days)-injected hyperthyroid rats, control normal rats, and thyroidectomized rats. The enzyme activities in both nuclear fractions were assayed under the condition of dose-response linearity. When RNA polymerase I activity in neuronal nuclei from control rats was expressed as 100%, the activities from T3-injected and hypothyroid rats were 112.3 +/- 3.4% (n = 5, P less than .05) and 86.9 +/- 3.5% (n = 5, P less than .05), respectively. The increase in the enzyme activities were parallel to the increase in T3 content in neuronal nuclei among the groups. Glial nuclear RNA polymerase I showed the same tendency in response to T3, although the enzyme activity was smaller than from neuronal nuclei. RNA polymerase II, however, showed no significant change in response to altered T3 levels. The existence of numerous receptors and an induction of increased RNA polymerase I activity by T3 in neuronal nuclei raise the possibility that thyroid hormone through a NT3R pathway in the cerebral cortex of even the mature rat brain.

Triiodothyronine effects on RNA polymerase activities in isolated neuronal and glial nuclei of the mature rat brain cortex.

In the‘template-engaged’form activity of RNA polymerase I in nuclei normal rat livers, about 40% of the activity was found to be influenced by thyroid hormones and glucocorticoids and can be eliminated by 1h treatment with cycloheximide. On the other hand, the ‘free’ form activity of the enzyme rat liver nuclei and the total amount of the enzyme in nucleoli were scarcely affected by these hormones. These results suggest that a considerable portion of rRNA synthesis in rat liver nucleoli is regulated by the initiation rDNA transcription, probably with the participation of glucocorticoid-and thyroid hormone-dependent short-lived protein (s). When somatotropin, in addition to glucocorticoids and thyroid hormones, was depleted by hypophysectomy, the‘template-engaged’form activity of RNA polymerase I was reduced to 3.0% of the normal level. The activity was restored completely by a single injection of triiodothyronine plus somatotropin, but not by injections of the individual hormones. From these results, it is likely that the stimulating effects of glucocorticoids and thyroid hormones on rRNA synthesis are maintained by the action of somatotropin which seems to occur in a different mechanism from those of the former two hormones. In contrast to these hormones, androgen showed no acute effect on‘template-engaged’form activity of RNA polymerase I in liver nuclei.

In Vivo Effects of Thyroid Hormone, Glucocorticoid, Androgen, and Somatotropin on RNA Polymerase I Activities in Isolated Liver Nuclei

After having defined reliable assay conditions for free and template-bound RNA polymerase I activities, we examined in detail the effect of thyroid hormone in vivo on nucleolar RNA polymerase I in rat liver. The enzyme activity decreased markedly in the hypothyroid state produced by either thyroidectomy, hypophysectomy or by administration of propylthiouracil (PTU). One large dose (25 microgram/100 g body weight) of T3 given intraperitoneally caused a rapid recovery of the enzyme activity within 24 h either in the presence or absence of the pituitary. The enzyme activity of both free and chromatin-bound enzyme changed in parallel throughout. A similar recovery was noted after daily subcutaneous injections of a physiological dose (1 microgram/100 g body weight) of T3 for about 3 weeks. Analyses of subfractions of the enzyme indicated that only IB-type enzyme (the enzyme fraction containing the whole set of subunits) changed in response to thyroid hormone action, also irrespective of the presence or absence of the pituitary. The possibility that thyroid hormone is a major physiological regulator of the synthesis of RNA polymerase I in rat liver is discussed.

Yokio Mishima

Papers

Enhancement by Thyroid Hormone of Free and Bound RNA Polymerase IB Activities in Rat Liver Nucleoli