Enhancing effect of magnesium ion on cell-free synthesis of read-through protein of bacteriophage Qbeta

Polyamines enhance synthesis of the RNA polymerase sigma 38 subunit by suppression of an amber termination codon in the open reading frame

The mechanisms by which polyamines stimulate synthesis of the RNA polymerase sigma(38) subunit in Escherichia coli were studied. Polyamine stimulation was observed only in strains in which the 33rd codon of RpoS mRNA is a UAG termination codon instead of a CAG codon for glutamine in wild-type E. coli. Readthrough of the termination codon by Gln-tRNA(supE) was stimulated by polyamines. This stimulation was found to be caused by an increase in both the level of suppressor tRNA(supE) and the binding affinity of Gln-tRNA(supE) for ribosomes. The stimulatory effect was observed with a UAG termination codon but not with UGA and UAA codons. Readthrough of the UAG termination codon at the 270th amino acid position of RpoS mRNA was also stimulated by polyamines, indicating that polyamines stimulate readthrough of a UAG codon regardless of its location within the RpoS mRNA. When cell viability of an E. coli strain having a termination codon in the 33rd position of RpoS mRNA was compared using cells cultured with or without putrescine, it was higher in cells cultured with putrescine than in cells cultured without putrescine. The level of sigma(38) subunit in the cells cultured with putrescine was higher than that in cells cultured without putrescine on days 2, 4, and 8, but the level of sigma(70) subunit was almost the same in cells cultured with or without putrescine. These results confirm that elevated expression of the rpoS gene is important for cell viability at late stationary phase.

Polyamine requirement for efficient translation of amber codons in vivo

Multiplication of several amber mutants of bacteriophage T7 was decreased in two polyamine-deficient mutants of Escherichia coli K-12 carrying amber suppressors, relative to the multiplication of wild type bacteriophage T7 in the same hosts. In contrast the same T7 amber bacteriophages multiplied well in these strains when supplemented with polyamines. The requirement for polyamines for optimal translation of amber codons in vivo was confirmed by showing that infection of polyamine-depleted E. coli with bacteriophage T7 carrying an amber mutation in gene 1 resulted in an increased accumulation of the amber fragment of the gene 1 protein and a decreased accumulation of the full-length gene 1 protein compared with infection of an amine-supplemented culture. These results indicate that one important function of polyamines in vivo is concerned with protein translation and the protein-synthesizing ribosomal complex.

Differential stimulation by polyamines of phage RNA-directed synthesis of proteins

The effect of polyamines on Q beta and MS2 phage RNA-directed synthesis of three kinds of protein in an Escherichia coli cell-free system has been studied. With both phage RNAs, the degree of stimulation of protein synthesis by spermidine was in the order RNA replicase greater than A protein, while the synthesis of coat protein was not stimulated significantly by spermidine. The synthesis of RNA replicase was stimulated by 1 mM spermidine approx. 8-fold. From the results of Q beta RNA direct alanyl-tRNA and seryl-tRNA binding to ribosomes and initiation dipeptide synthesis, it is suggested that the preferential stimulation of the synthesis of RNA replicase by spermidine is due at least partially to the stimulation of the initiation of RNA replicase synthesis.

Effects of (hydroxymethyl)trimethylpsoralen on structure and function of bacteriophage MS2 ribonucleic acid

Treatment of bacteriophage MS2 with 4'-(hydroxymethyl)-4,5',8-trimethylpsoralen and 360-nm light caused a dose-dependent decline in the infectivity of the virus. Covalent photobinding of a single psoralen molecule on the phage genome was a lethal event. Ribonucleic acid (RNA) extracted from psoralen and light-treated virus had a dose-dependent 385-nm fluorescence emission but was unaltered in its physical properties compared to control RNA samples. Phage adsorption and penetration in Escherichia coli host cells were unaffected, but in vivo replication of the treated virus was affected to the same extent as infectivity. The cell-free translational activity of the MS2 RNA was also severely reduced after psoralen and light treatment of the phage. Examination of the in vitro translation products revealed that the synthesis of the viral replicase protein was most substantially affected. Psoralen treatment of purified, protein-free MS2 RNA promoted an even greater reduction in cell-free synthesis of all viral proteins. This difference in translational function was consistent with the observation that virion-free RNA bound approximately 4 times as much psoralen as did RNA treated within the phage capsid. It was concluded that the replicase gene is the most sensitive region of the viral RNA molecule for psoralen binding.

A UGA termination suppression tRNATrp active in rabbit reticulocytes

A tRNATrp was purified from rabbit reticulocytes which suppresses the UGA termination codon of beta-haemoglobin mRNA. Evidence is presented that the beta-haemoglobin readthrough protein is found in reticulocyte translations and intact cells. Some natural readthrough proteins perform essential functions; they are synthesised through suppression of UGA or UAG but not UAA termination codons.