The biosynthetic rates for both the transferrin receptor (TfR) and ferritin are regulated by iron. An iron-responsive element (IRE) in the 5′ untranslated portion of the ferritin messenger RNA (mRNA) mediates iron-dependent control of its translation. In this report the 3′ untranslated region of the mRNA for the human TfR was shown to be necessary and sufficient for iron-dependent control of mRNA levels. Deletion studies identified a 678-nucleotide fragment of the TfR complementary DNA that is critical for this iron regulation. Five potential stem-loops that resemble the ferritin IRE are contained within the region critical for TfR regulation. Each of two of the five TfR elements was independently inserted into the 5′ untranslated region of an indicator gene transcript. In this location they conferred iron regulation of translation. Thus, an mRNA element has been implicated in the mediation of distinct regulatory phenomena dependent on the context of the element within the transcript.

Thymidylate synthase (TS; 5,10-methylenetetrahydrofolate:dUMP C-methyltransferase, EC 2.1.1.45) is essential for the de novo synthesis of thymidylate, a precursor of DNA. Previous studies have shown that the cellular level of this protein is regulated at both the transcriptional and posttranscriptional levels. The regulation of human TS mRNA translation was studied in vitro with a rabbit reticulocyte lysate system. The addition of purified human recombinant TS protein to in vitro translation reactions inhibited translation of TS mRNA. This inhibition was specific in that recombinant TS protein had no effect on the in vitro translation of mRNA for human chromogranin A, human folate receptor, preplacental lactogen, or total yeast RNA. The inclusion of dUMP, 5-fluoro-dUMP, or 5,10-methylene-tetrahydrofolate in in vitro translation reactions completely relieved the inhibition of TS mRNA translation by TS protein. Gel retardation assays confirmed a specific interaction between TS protein and its corresponding mRNA but not with unrelated mRNAs, including human placenta, human beta-actin, and yeast tRNA. These studies suggest that translation of TS mRNA is controlled by its own protein end product, TS, in an autoregulatory manner.

Hepatitis D virus (HDV) is the cause of an unusually severe form of liver disease with distinct histologic features (morula cell) that occurs throughout northern South America and certain other areas of the world. Clinical studies of HDV disease worldwide indicate that there is, in fact, a wide variation in pathogenesis, and the reasons for these differences are presently unknown. One possible explanation is that factors associated with the viral genotype are determinants of HDV pathogenesis. In this study, nucleic acid sequences were determined for three different northern South American HDV isolates which were obtained from individuals with severe disease or a family history of severe disease, in areas that are hyperendemic for this disease pattern. The sequences of these three isolates are very similar to one another but only distantly related to other published HDV sequences. Comparison of the sequence of a semiconserved region from a total of 14 isolates indicates that there are at least three HDV genotypes. Most published HDV sequences, including those from North America, Europe, the Middle East, the South Pacific, and Asia, belong to a single genotype which may have some geographically based subtypes. A single Japanese isolate is the sole representative of a second HDV genotype. The South American sequences reported here constitute a third genotype. The association of a particular genotype with the severe form of type D hepatitis that occurs in northern South America supports the hypothesis that HDV genetic factors are important determinants in the pathogenesis of type D hepatitis.

The level of mRNA encoding the transferrin receptor (TfR) is regulated by iron, and this regulation is mediated by a portion of the 3' untranslated region (UTR) of the TfR transcript. This portion of 3' UTR of the human TfR mRNA contains five RNA elements that have structural similarity to the iron-responsive element (IRE) found as a single copy in the 5' UTR of the mRNA for ferritin, whose translation is regulated by iron. Moreover, five very similar elements are also contained in the 3' UTR of the chicken TfR mRNA. Cytosolic extracts of human cell lines are shown by a gel shift assay involving RNase T1 protection to contain an IRE-binding protein capable of specific interaction with the human TfR 3' UTR. When the protecting protein is removed, the protected RNA can be digested with RNase T1 to yield oligoribonucleotide fragments characteristic of two of the IREs contained in the TfR 3' UTR. As judged by cross-competition experiments, the same IRE-binding protein interacts with the ferritin IRE. The apparent affinity of RNA sequence elements for the IRE-binding protein is shown to depend upon the sequence of the RNA. A comprehensive secondary structure for the regulatory region of the TfR mRNA is proposed based on the experimentally demonstrated presence of at least two IRE-like structural elements.