The modified nucleoside 2-thiocytidine (s 2 C) has so far been found in tRNA from organisms belonging to the phylogenetic domains Archaea and Bacteria. In the bacteria Escherichia coli and Salmonella enterica serovar
The life of aerobes is dependent on iron and oxygen for efficient bioenergetics. Due to potential risks associated with iron/oxygen chemistry, iron acquisition, concentration, storage, utilization, and efflux are tightly regulated in the cell. A central role in regulating iron/oxygen chemistry in animals is played by mRNA translation or turnover via the iron responsive element (IRE)/iron regulatory protein (IRP) system. The IRE family is composed of three-dimensional RNA structures located in 3' or 5' untranslated regions of mRNA. To date, there are 11 different IRE mRNAs in the family, regulated through translation initiation or mRNA stability. Iron or oxidant stimuli induce a set of graded responses related to mRNA-specific IRE substructures, indicated by differential responses to iron in vivo and binding IRPs in vitro. Molecular effects of phosphorylation, iron and oxygen remain to be added to the structural information of the IRE-RNA and IRP repressor in the regulatory complex.
Frameshift mutations can be suppressed by a variety of differently acting external suppressors. The +1 frameshift mutation hisC3072, which has an extra G in a run of Gs, is corrected by the external suppressor mutation sufF44. We have shown that sufF44 and five additional allelic suppressor mutations are located in the gene argU coding for the minor tRNA
The ribosomal 50S subunit protein L9, encoded by the gene rplI, is an elongated protein with an ␣-helix connecting the N-and C-terminal globular domains. We isolated rplI mutants that suppress the ؉1 frameshift mutation hisC3072 in Salmonella enterica serovar Typhimurium. These mutants have amino acid substitutions in the N-terminal domain (G24D) or in the C-terminal domain (I94S, A102D, G126V, and F132S) of L9. In addition, different one-base deletions in rplI altered either the final portion of the C terminus or removed the C-terminal domain with or without the connecting ␣-helix. An alanine-to-proline substitution at position 59 (A59P), which breaks the ␣-helix between the globular domains, induced ؉1 frameshifting, suggesting that the geometrical relationship between the N and C domains is important to maintain the reading frame. Except for the alterations G126V in the C terminus and A59P in the connecting ␣-helix, our results confirm earlier results obtained by using the phage T4 gene 60-based system to monitor bypassing. The way rplI mutations suppress various frameshift mutations suggests that bypassing of many codons from several takeoff and landing sites occurred instead of a specific frameshift forward at overlapping codons.
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