Effect of a purified initiation factor F3 (B) on the selection of ribosomal binding sites on phage MS2 RNA

Hueneman K, Bartram J, Stepanchick E, R. Bennett J, E. Culver-Cochran A, Bolanos LC, Uible E, Choi K, Wunderlich M, Perentesis JP, M. Chlon T, Filippi MD, Starczynowski DT. The deubiquitinase USP15 modulates cellular redox and is a therapeutic target in acute myeloid leukemia

Ubiquitin-specific peptidase 15 (USP15) is a deubiquitinating enzyme implicated in critical cellular and oncogenic processes. We report that USP15 mRNA and protein are overexpressed in human acute myeloid leukemia (AML) as compared to normal hematopoietic progenitor cells. This high expression of USP15 in AML correlates with KEAP1 protein and suppression of NRF2. Knockdown or deletion of USP15 in human and mouse AML models significantly impairs leukemic progenitor function and viability and de-represses an antioxidant response through the KEAP1-NRF2 axis. Inhibition of USP15 and subsequent activation of NRF2 leads to redox perturbations in AML cells, coincident with impaired leukemic cell function. In contrast, USP15 is dispensable for human and mouse normal hematopoietic cells in vitro and in vivo. A preclinical small-molecule inhibitor of USP15 induced the KEAP1-NRF2 axis and impaired AML cell function, suggesting that targeting USP15 catalytic function can suppress AML. Based on these findings, we report that USP15 drives AML cell function, in part, by suppressing a critical oxidative stress sensor mechanism and permitting an aberrant redox state. Furthermore, we postulate that inhibition of USP15 activity with small molecule inhibitors will selectively impair leukemic progenitor cells by re-engaging homeostatic redox responses while sparing normal hematopoiesis.

Influence of mRNA secondary structure on binding and migration of 40S ribosomal subunits

Reovirus messenger RNA was modified by reaction with bisulfite (in denaturing conditions) or by incorporation of IMP in place of GMP, thereby irreversibly unfolding the mRNA. Messenger RNA in which the secondary structure was weakened or abolished retained the ability to bind to wheat germ ribosomes, suggesting that conformational features around the AUG codon are not required for ribosome recognition of mRNA. Ribosomes were not able to attach (directly) to spurious internal sites, even in extensively unfolded RNA, indicating that the monocistronic character of eucaryotic messages (in which initiation is limited to a single 5' proximal site) is not simply due to conformational masking of all the internal AUG codons. The secondary structure in eucaryotic messages does contribute to the fidelity of the translation process, however, because when 40S ribosomal subunits were incubated with denatured mRNA they failed to stop at the 5' proximal AUG codon. Extensive migration beyond the 5' region occurred when 40S ribosomes (in the absence of 60S subunits) attached to unfolded mRNA, implying that the secondary structure in native mRNA facilitates correct translation by impeding migration of 40S subunits beyond the 5' proximal initiation region. Secondary structure in mRNA may also modulate the efficiency of translation. Studies with BrUMP-substituted mRNA, in which the secondary structure is enhanced, suggested that the efficiency of mRNA binding to ribosomes decreases as the stability of the secondary structure increases.

Isolation of 30S and 50S active ribosomal subunits of Bacillus subtilis, Marburg strain

Active 30S and 50S ribosomal subunits were isolated from Bacillus subtilis. These subunits were able to perform not only protein synthesis in the presence of artificial or natural messenger ribonucleic acid but also the specific functions characteristic of each of the subunits. Thus the 30S subunits alone are able to bind formyl-methionyl-transfer ribonucleic acid, and the 50S subunits carry the peptidyl transferase activity.

Determinant of cistron specificity in bacterial ribosomes

The sequence of the 3'-terminus of 16S RNA from different bacteria has been determined. Complementarity relationships between this sequence and a purine-rich tract in the ribosome binding site of different bacterial mRNAs suggest that the 3'-end of 16S RNA determines the intrinsic capacity of ribosomes to translate a particular cistron.

Escherichia coli initiation factor IF3 binding to AUG and AUG-containing single strands and hairpin loops, and nonspecific binding to polymers

Nitrocellulose filter binding and equilibrium dialysis detected the binding of Escherichia coli initiation factor IF3 to AUG, An UGUm single strands and hairpin loops, poly(A,U,G), poly(U), and f2 RNA. No binding was detected for GUA, A8 U, or the hairpin loop A5 GC5 U5. AUG-specific binding, per nucleotide, is strong; nonspecific binding, per nucleotide, is weak.

Mitochondrial and cytoplasmic ribosomes. Distinguishing characteristics and a requirement for the homologous ribosomal salt-extractable fraction for protein synthesis

1. Mitochondrial and cytoplasmic ribosomes of Euglena gracilis differ in their total RNA and protein content. 2. Mitochondrial ribosomes dissociate to subunits at higher Mg(2+) concentrations than do cytoplasmic ribosomes. 3. A separable 5S RNA is obtained from cytoplasmic and chloroplast ribosomes, but not from mitochondrial ribosomes. 4. For protein-synthesizing activity with a natural mRNA, mitochondrial ribosomes use tRNA from any cell compartment and are partly active with supernatant enzymes from cytoplasm. Cytoplasmic ribosomes are partly active with enzymes and tRNA from mitochondria or chloroplasts. 5. Both mitochondrial and cytoplasmic ribosomes show high specificity for the homologous salt-extractable ribosomal fraction for protein-synthesizing activity.

Messenger selection by bacterial ribosomes

The counterpart of Escherichia coli initiation factor 3(IF-3) was isolated from Caulobacter crescentus, purified to homogeneity, and used in comparative studies on in vitro translation of RNA from the C. crescentus RNA phage Cb5 and of coliphage MS2 RNA. The two phage RNAs are similar in physical properties and analogous in genetic content. The factor, C-IF-3, substitutes for E. coli IF-3 and promotes correct translation of MS2 RNA by E. coli ribosomes. Conversely, E. coli IF-3 substitutes for C-IF-3 in translation of Cb5 RNA by C. crescentus ribosomes. However, each phage RNA could be translated only by host ribosomes or by mixed ribosomes containing the host 30S subunit. C-IF-3 dissociates C. crescentus and E. coli 70S ribosomes into subunits. It binds phage, ribosomal, and, less efficiently, transfer RNA.

Initiation factor IF-3-dependent binding of Escherichia coli ribosomes and N-formylmethionine transfer-RNA to rabbit globin messenger

An initiation complex has been formed in high yields from E. coli ribosomes, 9S messenger RNA for rabbit hemoglobin, and N-formylmethionine-tRNA. Initiation factor IF-3 is required for the binding and puromycin is required for the release of fMet. Valyl-tRNA fails to bind to the second codon, whereas a mixture of 15 aminoacyl-tRNAs promotes incorporation. Together with quantitative data, the findings suggest that IF-3 directs the ribosomes to an AUG codon on one of the two globin messengers, at a site that is different from the normal starting point for globin synthesis.