SUMMARY Formation of mRNA 3′ ends in eukaryotes requires the interaction of transacting factors with cis-acting signal elements on the RNA precursor by two distinct mechanisms, one for the cleavage of most replication-dependent histone transcripts and the other for cleavage and polyadenylation of the majority of eukaryotic mRNAs. Most of the basic factors have now been identified, as well as some of the key protein-protein and RNA-protein interactions. This processing can be regulated by changing the levels or activity of basic factors or by using activators and repressors, many of which are components of the splicing machinery. These regulatory mechanisms act during differentiation, progression through the cell cycle, or viral infections. Recent findings suggest that the association of cleavage/polyadenylation factors with the transcriptional complex via the carboxyl-terminal domain of the RNA polymerase II (Pol II) large subunit is the means by which the cell restricts polyadenylation to Pol II transcripts. The processing of 3′ ends is also important for transcription termination downstream of cleavage sites and for assembly of an export-competent mRNA. The progress of the last few years points to a remarkable coordination and cooperativity in the steps leading to the appearance of translatable mRNA in the cytoplasm.

SUMMARYA sequence of 5987 nucleotides is reported for the RNA of potato leafroll luteovirus (PLRV). The sequence contains six large open reading flames, and non-coding regions of 174 nucleotides at the 5' end, 141 nucleotides at the 3' end and 197 nucleotides between two large blocks of coding sequences. The 5' coding region encodes two polypeptides of 28 000 (28K) and 70K which overlap in different reading frames and circumstantial evidence suggests that the third open reading frame in the 5' block is translated by frameshift readthrough near the end of the 70K polypeptide to give a l18K polypeptide. The C-terminal part of the l18K protein contains the consensus sequence for RNA-dependent RNA polymerases. In vitro translation of PLRV RNA resulted in the synthesis mainly of 28K and 70K polypeptides and the largest product made was about 125K; these sizes are similar to those predicted for the translation products of the 5' block of coding sequence. The 3' block of coding sequence codes for three polypeptides: a 23K coat protein, a 17K polypeptide which is encoded in a different frame, and a 53K polypeptide which immediately follows the coat protein coding sequence, and is in the same reading frame. Circumstantial evidence suggests that the 53K polypeptide is translated by readthrough of the amber termination codon of the coat protein gene. The amino acid sequences encoded by the 3' block of coding sequence show many similarities with analogous polypeptides translated from the nucleotide sequences of RNA of barley yellow dwarf virus, PAV strain (BYDV) and,

The pBEVY (bi-directional expression vectors for yeast) plasmids were designed with constitutive and galactose-induced bi-directional promoters to direct the expression of multiple proteins in Saccharomyces cerevisiae . Using human estrogen receptor as a test gene, relatively balanced expression levels from each side of a bi-directional promoter were observed. Expression of a functional heterodimeric transcription factor composed of human aryl hydrocarbon receptor (Ahr) and aryl hydrocarbon receptor nuclear translocator (Arnt) proteins was accomplished using a single pBEVY plasmid. Previous studies suggest that inhibitory cross-talk between the estrogen receptor and the Ahr/Arnt complex may occur and that Hsp90-Ahr complex formation is important for Ahr-mediated signal transduction. Evidence for functional interaction among these proteins was investigated using pBEVY plasmids in a yeast system. No inhibitory cross-talk was observed in signaling assays performed with yeast that co-expressed Ahr, Arnt and estrogen receptor. In contrast, Ahr/Arnt-mediated signal transduction was reduced by 80% in a temperature-sensitive Hsp90 strain grown under non-permissive conditions. We conclude that pBEVY plasmids facilitate the examination of multiple protein interactions in yeast model systems.

Ribosomal protein synthesis ceases upon maturation of Xenopus oocytes. We find that this cessation results from the dissociation of ribosomal protein mRNAs from polysomes and is accompanied by the deadenylation of these transcripts. A synthetic mRNA encoding ribosomal protein LI, microinjected into stage VI oocytes, is deadenylated and released from polysomes upon maturation. Our results indicate that sequences located within 387 bp of the 3' terminus of LI mRNA direct both the deadenylation and polysomal release of this ribosomal protein mRNA. The proper translational regulation of an exogenous ribosomal protein mRNA in microinjected oocytes provides a basis for determining the sequence specificity for the differential utilization of maternal mRNAs during oocyte maturation. The synthesis of ribosomal proteins during Xenopus oo genesis contributes to the production of a maternal ribosome pool that is sufficient to support protein synthesis throughout embryogenesis. This elevated accumulation of ribosomal proteins is due largely to the preferential translation of ribosomal protein mRNAs (rp-mRNAs) in vitellogenic stage III oocytes w^hen ribosomal proteins comprise -20% of total protein synthesis (Dixon and Ford 1982;Baum and Wormington 1985; CardinaH et al. 1987). In contrast, synthesis of the majority of ribosomal proteins during embryogenesis is not observed until de velopment of the tail-bud embryo. Previous studies by Pierandrei-Amaldi and colleagues (1982) and Baum and Wormington (1985) have shoMrn that tv^o principal factors contribute to the absence of ribosomal protein synthesis in early embryonic development. First, ma ternal rp-mRNAs are degraded following fertilization. Second, although zygotic rp-mRNAs begin to accumu late during early gastrulation, these transcripts are ex cluded from polysomes until the tail-bud stage. Thus, the onset of ribosomal protein synthesis in both oo genesis and embryogenesis is regulated by selective rpmRNA utilization.We are interested in determining the mechanisms that direct the transition from active ribosomal protein syn thesis during oogenesis to the translational inactivity and instability of these maternal mRNAs follov^ing fer tilization. The process of oocyte maturation comprises several events likely to participate in the cessation of ribosomal subunit assembly. The induction of matura tion initiates a series of physiological changes, including the resumption of meiosis, breakdown of the nuclear en velope, chromosome condensation, and repression of transcription by all three classes of nuclear RNA poly merase (reviewed in Mailer 1985). The absence of both rp-mRNA and rRNA transcription and precursor rRNA processing (Busby and Reeder 1982), in particular, pre cludes the assembly of ribosomal subunits in mature oo cytes.In this paper we have examined the regulation of ribo somal protein synthesis during oocyte maturation. Our results indicate that the majority of ribosomal proteins are not synthesized in mature oocytes, despite a twofold increase in overall prote...