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Abstract. We compared the surface envelope glycoprotein distribution and the budding polarity of four RNA viruses in Fischer rat thyroid (FRT) cells and in CaCo-2 cells derived from a human colon carcinoma . Whereas both FRT and CaCo-2 cells sort similarly influenza hemagglutinin and vesicular stomatitis virus (VSV) G protein, respectively, to apical and basolateral membrane domains, they differ in their handling of two togaviruses, Sindbis and Semliki Forest virus (SFV) . By conventional EM Sindbis virus and SFV were shown to bud apically in FRT cells and basolaterally in CaCo-2 cells. Consistent with this finding,

We have isolated the entire gene for rat thyroglobulin, the precursor for thyroid hormone biosynthesis. The gene is at least 170,000 base pairs (bp) long; 9000 bp of coding information are distributed in 42 exons of homogeneous size (150-200 bp) except for two exons of 1100 and 620 bp. The sequences coding for two major thyroxine-forming sites are localized in exons 2 and 39. These two sequences do not show any homology either at the DNA or at the protein-sequence level, even though they code for sites highly specialized for the same function. Furthermore, both the 3' and the 5' end of the thyroglobulin structural gene appear to be made of repetitive units, which again do not show any homology. On the basis of these observations, we propose that the thyroglobulin gene arose by shuffling of at least two segments, with different evolutionary histories, each of which already contained introns.completely unrelated sequence organization, as shown by the presence of two different repeated units. MATERIALS AND METHODSLibraries and Vectors. Two rat genomic libraries have been used in this study: the Hae III/Alu I partial-digest library was kindly provided by J. Bonner (15); the EcoRI partial-digest library was constructed in our laboratory as described (12). Screenings of libraries, purification of X phage DNA, and probe labeling by nick-translation were carried out by published protocols (16). R loops were obtained and analyzed as described by Davis et al. (17). Repetitive elements were mapped in the recombinant X phages (12) and DNA was sequenced (18) as described.

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We have mesured the synthesis of thyroglobulin in two differentiated cell lines, FRTL-5 and FRTL-424, and two transformed thyroid cell lines, FRA and 1-5G. The untransformed cells actively synthesized and exported thyroglobulin in the medium: however, the FRTL-5 cell line synthesized seven times less thyroglobulin than the FRTL-424 cell line, even though both cell types contained equal amounts of functional thyroglobulin mRNA. In contrast the transformed cells expressed extremely low levels of thyroglobulin mRNA, even though there was no detectable change in gene structure or copy number as determined by Southern blot analysis. On the basis of these data we conclude that (a) the different levels of thyroglobulin synthesis in the two untransformed cell lines are due to stable post-transcriptional alterations in the biosynthesis of thyroglobulin and (b) the transformation of thyroid cells results in a substantial reduction in thyroglobulin gene expression.The biosynthesis of thyroglobulin is a distinctive feature of differentiated thyroid cells. This protein represents about 70% of the newly synthesized proteins of the tyroid gland in vivo [l]. Recently information has been obtained on the primary structure of the protein [la] and on the structural organization of the rat [2] and human [3] genes. The regulation of the expression of thyroglobulin gene, however, is still a difficult problem to approach. An ideal system for the investigation of the molecular steps of thyroglobulin biosynthesis would be a collection of cell strains defective in one or more steps of the thyroglobulin expression pathway. The availability of rat thyroid cell lines, normal and transformed, together with DNA probes, makes this aim realistic.In this report, as the first approach to the study of the mechanisms involved in the control of the thyroglobulin biosynthesis, we have quantified the expression of the thyroglobulin gene at the mRNA and protein levels in untransformed and transformed thyroid cell lines [4, 51. We suggest that the lower rate of expression of thyroglobulin in the FRTL-5 cell line is due to a stable post-transcriptional alteration in the biosynthesis of thyroglobulin. Morphological analysis of the two untransformed cell lines correlates well with the biochemical results. In addition we have measured the amount of thyroglobulin and thyroglobulin mRNA in two transformed cell lines derived from different thyroid tumours: thyroglobulin was not detectable in either cell line and only the FRA cells produced detectable levels of thyroglobulin mRNA. ~~