These studies identify an organic solute transporter (OST) that is generated when two novel gene products are co-expressed, namely human OST␣ and OST␤ or mouse OST␣ and OST␤. The results also demonstrate that the mammalian proteins are functionally complemented by evolutionarily divergent Ost␣-Ost␤ proteins recently identified in the little skate, Raja erinacea, even though the latter exhibit only 25-41% predicted amino acid identity with the mammalian proteins. Human, mouse, and skate OST␣ proteins are predicted to contain seven transmembrane helices, whereas the OST␤ sequences are predicted to have a single transmembrane helix. Human OST␣-OST␤ and mouse Ost␣-Ost␤ cDNAs were cloned from liver mRNA, sequenced, expressed in Xenopus laevis oocytes, and tested for their ability to functionally complement the corresponding skate proteins by measuring transport of [ 3 H]estrone 3-sulfate. None of the proteins elicited a transport signal when expressed individually in oocytes; however, all nine OST␣-OST␤ combinations (i.e. OST␣-OST␤ pairs from human, mouse, or skate) generated robust estrone 3-sulfate transport activity. Transport was sodium-independent, saturable, and inhibited by other steroids and anionic drugs. Human and mouse OST␣-OST␤ also were able to mediate transport of taurocholate, digoxin, and prostaglandin E 2 but not of estradiol 17␤-D-glucuronide or p-aminohippurate. OST␣ and OST␤ were able to reach the oocyte plasma membrane when expressed either individually or in pairs, indicating that co-expression is not required for proper membrane targeting. Interestingly, OST␣ and OST␤ mRNAs were highly expressed and widely distributed in human tissues, with the highest levels occurring in the testis, colon, liver, small intestine, kidney, ovary, and adrenal gland.

Dynamic phosphorylation of the RNA polymerase II CTD repeats (YS2PTS5PS7) is coupled to transcription and may act as a “code” that controls mRNA synthesis and processing. To examine the "code" in budding yeast, we mapped genome-wide CTD S2, 5 and 7 phosphorylations (PO4) and compared them with the CTD-associated termination factors, Nrd1 and Pcf11. CTD-PO4 dynamics are not scaled to the size of the gene. At 5’ ends, the onset of S2-PO4 is delayed by about 450 bases relative to S5-PO4, regardless of gene length. Phospho-CTD dynamics are gene-specific, with high S5/7-PO4 at the 5' end being characteristic of well-expressed genes with nucleosome-occupied promoters. Furthermore, the CTD kinases Kin28 and Ctk1 profoundly affect pol II distribution along genes in a highly gene-specific way. The "code" is therefore written differently on different genes, probably under the control of promoters. S7-PO4 is enriched on introns and at sites of Nrd1 accumulation suggesting that this modification may function in splicing and Nrd1 recruitment. Nrd1 and Pcf11 frequently co-localized, suggesting functional overlap between these terminators. Surprisingly, Pcf11 is also recruited to centromeres and pol III transcribed genes.

Uptake of organic solutes and xenobiotics by mammalian cells is mediated by ATP-independent transporters, and four families of transporters have now been identified. To search for novel organic solute transporters, a liver cDNA library from an evolutionarily primitive marine vertebrate, the little skate Raja erinacea, was screened for taurocholate transport activity by using Xenopus laevis oocytes. In contrast to the organic anion transporters identified to date, a transport activity was identified in this library that required the coexpression of two distinct gene products, termed organic solute transporter ␣ and ␤ (Ost␣, Ost␤). Ost␣ cDNA encodes for a protein of 352 aa and seven putative transmembrane (TM) domains. Ost␤ contains 182 aa and has at least one and perhaps two TM domains. There is no significant sequence identity between Ost␣ and Ost␤, and only low identity with sequences in the databases; however, Ost␣ bears a resemblance to some G protein-coupled receptors, and Ost␤ exhibits 22% amino acid identity with the C-terminal TM and intracellular domains of protocadherin-␥, a cell surface glycoprotein. Xenopus oocytes injected with the cRNA for both Ost␣ and Ost␤, but not each separately, were able to take up taurocholate, estrone sulfate, digoxin, and prostaglandin E 2, but not p-aminohippurate or Sdinitrophenyl glutathione. Transport was sodium-independent, saturable, and inhibited by organic anions and steroids, including the major skate bile salt, scymnol sulfate. These results identify an organic anion transporter composed of a putative seven-helix TM protein and an ancillary membrane polypeptide. E pithelial cells continuously extract large amounts of organic solutes, drugs, and other xenobiotics from circulating blood plasma. Some of the transporters responsible for this drug clearance recently have been characterized at the molecular level and include four families of ATP-independent transporters: the Na ϩ -coupled bile acid transporters (NTCPs), the Na ϩ -independent organic anion transporting polypeptides (OATPs), the organic anion transporters (OATs), and the organic cation transporters (OCTs) (1-5). The first member of each of these families was identified by expression cloning in Xenopus laevis oocytes, and additional members subsequently have been identified by homology screening. All of these transporters consist of single polypeptides, which, when expressed in heterologous systems, are able to mediate organic solute transport.Because of the large number of endogenous and exogenous compounds that must be transported by the liver, kidney, intestine, and other tissues, it is likely that other transporters and transporter families also exist, but have not yet been described (5). In an attempt to identify novel organic anion transport proteins we used a comparative approach, screening a liver cDNA library from an evolutionarily ancient vertebrate species, the little skate Raja erinacea. This elasmobranch is thought to have evolved 200 million years ago, yet displays many physiological features...