Yeast Pab1 interacts with Rna15 and participates in the control of the poly(A) tail length in vitro.

In Saccharomyces cerevisiae, the single poly(A) binding protein, Pab1, is the major ribonucleoprotein associated with the poly(A) tails of mRNAs in both the nucleus and the cytoplasm. We found that Pab1 interacts with Rna15 in two-hybrid assays and in coimmunoprecipitation experiments. Overexpression of PAB1 partially but specifically suppressed the rna15-2 mutation in vivo. RNA15 codes for a component of the cleavage and polyadenylation factor CF I, one of the four factors needed for pre-mRNA 3'-end processing. We show that Pab1 and CF I copurify in anion-exchange chromatography. These data suggest that Pab1 is physically associated with CF I. Extracts from a thermosensitive pab1 mutant and from a wild-type strain immunoneutralized for Pab1 showed normal cleavage activity but a large increase in poly(A) tail length. A normal tail length was restored by adding recombinant Pab1 to the mutant extract. The longer poly(A) tails were not due to an inhibition of exonuclease activities. Pab1 has previously been implicated in the regulation of translation initiation and in cytoplasmic mRNA stability. Our data indicate that Pab1 is also a part of the 3'-end RNA-processing complex and thus participates in the control of the poly(A) tail lengths during the polyadenylation reaction.

Multiple group II self-splicing introns in mobile DNA from Escherichia coli.

By PCR (polymerase chain reaction) amplification and cloning, we have identified four group II self-splicing introns encoding proteins related to reverse transcriptases in natural Escherichia coli isolates belonging to the ECOR collection. One intron, IntD, interrupts a DNA sequence virtually identical to that of the previously described IS3411 Insertion Sequence. A second intron, IntC, is located within an open reading frame that is closely related to a reading frame in the T-DNA of Agrobacterium tumefaciens. Finally, introns IntA and IntB are inserted at two distinct sites in one of the Rhs elements of E. coli. A comparison of their open reading frames shows that the two Rhs introns are more closely related to each other than to any other known group II intron: this suggests that transposition of group II introns may occur preferentially in cis, along the same piece of DNA.

Organization of the yeast URA2 gene: identification of a defective dihydroorotase-like domain in the multifunctional carbamoylphosphate synthetase-aspartate transcarbamylase complex.

The 6636 bp of the yeast URA2 gene encoding the carbamoylphosphate synthetase-aspartate transcarbamylase complex have been sequenced. The protein is organized into four regions, three of which are functional domains as indicated previously by genetic analysis. The fourth domain corresponds to a defective dihydroorotase called DHOase-like. The URA2 gene complex with the same organization as the equivalent genes in higher eukaryotes suggests an evolution from a common ancestral gene.

The primary structure of the aspartate transcarbamylase region of the URA2 gene product in Saccharomyces cerevisiae. Features involved in activity and nuclear localization.

The yeast URA2 locus encodes a multifunctional protein which possesses the carbamylphosphate synthetase and aspartate transcarbamylase activities and which catalyzes the first two reactions of the pyrimidine pathway. We report here the nucleotide sequence of the central and the 3' region of this locus. The latter encodes that part of the multifunctional protein which has the aspartate transcarbamylase activity. The deduced amino acid sequence shows a high degree of homology with the known aspartate transcarbamylases of various organisms from Escherichia coli to mammals. The amino acid residues that have been shown to be involved in the catalytic site of the E. coli enzyme are all conserved suggesting that, in the more complex structure of the yeast protein, the catalytic sites are also located at subunit interfaces. There is also an important conservation of the amino acid pairs that, in E. coli, are implicated in intra- and interchain interactions. As well as the oligomeric structure suggested by these two features, the three-dimensional structure of the yeast enzyme must also be organized to account for the channeling of carbamylphosphate, from the carbamylphosphate synthetase catalytic site to that of aspartate transcarbamylase, and for the concomitant feedback inhibition of the two activities by the end product UTP. The URA2 gene product was shown to be localized in the nucleus. With the aim of identifying the regions that may be involved in this transport, we have determined by electron microscopy the subcellular distribution of aspartate transcarbamylase in three strains expressing different fragments of the URA2 locus. In the first strain the protein lacks 190 residues at the N terminus, but accumulates normally in the nucleus. In the second strain the protein lacks 382 residues in the central part and seems impaired in the nuclear transport process. In the third strain the 476-residue protein encoded by the 3' region of URA2 locus and catalyzing the aspartate transcarbamylase reaction is able by itself to migrate to and accumulate in the nucleus. This suggests that two regions are involved in the nuclear accumulation. On the basis of their conservation in analogous proteins of other eukaryotes and their similarity to sequences already identified as nuclear location signals, a sequence in the central region of the protein and two short sequences in the C-terminal region are good candidates for the nuclear location signal involved in the targeting of the URA2 product.