Conformational rearrangements upon start codon recognition in human 48S translation initiation complex

Selection of the translation start codon is a key step during protein synthesis in human cells. We obtained cryo-EM structures of human 48S initiation complexes and characterized the intermediates of codon recognition by kinetic methods using eIF1A as a reporter. Both approaches capture two distinct ribosome populations formed on an mRNA with a cognate AUG codon in the presence of eIF1, eIF1A, eIF2–GTP–Met-tRNAiMet, and eIF3. The ‘open’ 40S subunit conformation differs from the human 48S scanning complex and represents an intermediate preceding the codon recognition step. The ‘closed’ form is similar to reported structures of complexes from yeast and mammals formed upon codon recognition, except for the orientation of eIF1A, which is unique in our structure. Kinetic experiments show how various initiation factors mediate the population distribution of open and closed conformations until 60S subunit docking. Our results provide insights into the timing and structure of human translation initiation intermediates and suggest the differences in the mechanisms of start codon selection between mammals and yeast.

30 A TRANSCRIPTION ACTIVATOR-LIKE EFFECTOR NUCLEASE (TALEN)-MEDIATED UNIVERSAL GENE KNOCK-IN STRATEGY FOR MAMMARY GLANDS-SPECIFIC EXPRESSION OF RECOMBINANT PROTEINS IN DAIRY CATTLE

To harness the great capability of producing biologically active recombinant proteins with animal mammary glands, active research has been carried out in the past several decades to develop transgenic animals as bioreactors. However, when a transgene is introduced in the animal genome by random integration, the transgene tends to be subjected to epigenetic silencing, due to the so-called position effect from the chromatin environments surrounding the transgene integration sites, thereby resulting in low-level expression or total suppression. We report a universal transgenic strategy to knock in (KI) transgenes into the bovine β-casein gene locus allowing the expression of a transgene to be totally under the control of the endogenous regulatory sequences of the bovine β-casein gene. This universal KI strategy comprises two key components: one is the design of transcription activator-like effector nuclease (TALEN) constructs targeting the start codon region of bovine β-casein gene, and the other is the design of KI vectors in which a transgene of choice is flanked with homologous arms isolated from the ~500-bp bovine genomic DNAs immediately 5′ and 3′, respectively, of the translation start codon of the bovine β-casein gene. By using the human erythropoietin (hEPO) as the model transgene, we demonstrated that a transgene can be highly efficiently integrated immediately after the translation start codon of the bovine β-casein gene. In brief, the TALEN constructs were assembled by using the Golden Gate protocol. To KI the hEPO transgene, early passage (<5) of fibroblasts established from Holstein dairy cattle were cultured into full confluence in Dulbecco's modified Eagle medium (DMEM) supplemented with 10% fetal bovine serum (FBS), harvested with 0.25% trypsin-EDTA, and co-transfected with KI vector and the TALEN constructs by the Amaxa Nucleofector system. For each experiment, 106 cells were transfected with 5 μg of KI vector and 5 μg of TALEN constructs. After 72 h post-transfection, cells were harvested and subjected to limiting dilution to obtain single-cell derived colonies. To screen for single-cell derived colonies carrying the correctly KI of hEPO in the β-casein locus, we performed genomic PCR amplifying the genomic junctions created by the KI of hEPO gene into the bovine genome. We identified and established 2 hEPO transgenic bovine fibroblast cell lines after screening 10 single-cell derived colonies from the transfected cells (20%). The genotype of these 2 colonies was also confirmed by sequencing the PCR products. We have initiated the effort to produce hEPO transgenic cattle by somatic cell nuclear transfer (SCNT), and the animal cloning results will be reported at the conference.

Aquareovirus Effects Syncytiogenesis by Using a Novel Member of the FAST Protein Family Translated from a Noncanonical Translation Start Site

ABSTRACT As nonenveloped viruses, the aquareoviruses and orthoreoviruses are unusual in their ability to induce cell-cell fusion and syncytium formation. While an extraordinary family of fusion-associated small transmembrane (FAST) proteins is responsible for orthoreovirus syncytiogenesis, the basis for aquareovirus-induced syncytiogenesis is unknown. We now report that the S7 genome segment of an Atlantic salmon reovirus is polycistronic and uses a noncanonical CUG translation start codon to produce a 22-kDa integral membrane protein responsible for syncytiogenesis. The aquareovirus p22 protein represents a fourth distinct member of the FAST family with a unique repertoire and arrangement of structural motifs.

Functional Analyses of the Promoters in the Lantibiotic Mutacin II Biosynthetic Locus in Streptococcus mutans

ABSTRACT The lantibiotic bacteriocin mutacin II is produced by the group IIStreptococcus mutans. The mutacin II biosynthetic locus consists of seven genes, mutR, -A, -M, -T, -F, -E, and -G, organized as two operons. The mutAMTFEGoperon is transcribed from the mutA promoter 55 bp upstream of the translation start codon for MutA, while the mutRpromoter is 76 bp upstream of the mutR structural gene. Expression of the mutA promoter is regulated by the components of the growth medium, while the mutR promoter activity does not seem to be affected by these conditions. Inactivation of mutR abolishes transcription of the mutAoperon but does not affect its own promoter activity. The expressions of both mutA and mutR promoters are independent of the growth stage, while the production of mutacin II is only elevated at the early stationary phase. Taken together, these results suggest that expression of the mutacin operon is regulated by a complex system involving transcriptional and posttranscriptional or posttranslational controls.