Robert F. Goldberger

Prior studies to this article by Anfinsen, Epstein, and Goldberger established that the time required for synthesizing a chain of protein such as ribonuclease in the tissues of higher organisms would be approximately two minutes. Here, the three researchers noted that an enzyme system in the endoplasmic reticulum of cells that catalyzed the disulfide interchange reaction and which, when added to solutions of reduced ribonuclease, catalyzed the rapid formation of the native disulfide pairing in a period less than two minutes. Subsequent studies would find that such motile proteins as staphylococcal nuclease or myoglobin could undergo virtually complete renaturation in only a few seconds or less.

At the present time it is generally assumed that most, if not all, proteins possess well-defined structures. This assumption is based on several kinds of evidence: (1) the finding that many proteins have unique amino acid sequences, with specific intrachain disulfide bonds; (2) the ability to determine the three-dimensional configuration (tertiary structure) and, in some instances, even portions of the amino acid sequence of proteins such as myoglobin and hemoglobin by X-ray crystallographic methods; (3) the highly reproducible physical-chemical properties of a large number of proteins; and (4) the great degree of configurational specificity which appears necessary for enzymic activity. When one considers the complexity of the tertiary structure of a “native” protein, it seems reasonable to inquire into how a newly-made protein arrives at its three-dimensional configuration.

A new term, autogenous regulation, is used to describe a phenomenon that is not a new discovery but rather is newly appreciated as a mechanism common to a number of systems in both prokaryotic and eukaryotic organisms. In this mechanism the product of a structural gene regulates expression of the operon in which that structural gene resides. In many (perhaps all) cases, the regulatory gene product has several functions, since it may act not only as a regulatory protein but also as an enzyme, structural protein, or antibody, for example. In a few cases, this protein is the multimeric allosteric enzyme that catalyzes the first step of a metabolic pathway, gearing together the two most important mechanisms for controlling the biosynthesis of metabolites in bacterial cells-feedback inhibition and repression. Autogenous regulation may provide a mechanism for amplification of gene expression (84); for severe and prolonged inactivation of gene expression (85); for buffering the response of structural genes to changes in the environment (45, 52); and for maintaining a constant intracellular concentration of a protein, independent of cell size or growth rate (86). Thus, autogenous regulation provides the cell with means for accomplishing a number of different regulatory tasks, each suited to better satisfying the needs of the organism for its survival.

Livers of egg-laying species contain abundant mRNAs encoded by both estrogen-responsive and constitutively expressed genes. We have recently constructed cDNA clones from three members of the abundant mRNA class of hen liver. One of these mRNA species was identified as serum albumin mRNA, and another as vitellogenin mRNA. In this study we have identified the third member of the group as apo VLDLII mRNA. Hybridization analyses using cloned cDNA probes indicate that expression of the apo VLDLII gene in rooster liver, like that of the vitellogenin gene, is completely dependent upon the administration of estrogen. The apo VLDLII and vitellogenin genes appear to be the only genes capable of high rates of expression in the liver that exhibit such an exceptional response to the hormone. Administration of estrogen resulted in the appearance of both mRNA species within 30 min, followed by a rapid accumulation to several thousand copies per cell. Removal of the hormone caused a marked destabilization of both vitellogenin mRNA and apo VLDLII mRNA. In contrast, the absolute levels of serum albumin mRNA were unaffected by the hormone. Comparative studies on the structure and organization of these three genes may reveal elements involved in determining their rates of expression in the presence and absence of estrogen.