Excess of charged tRNA(Lys) maintains low levels of peptidyl-tRNA hydrolase in pth(Ts) mutants at a non-permissive temperature

Cellular changes have been monitored during the suppression, mediated by the overproduction of tRNA(Lys), of thermosensitivity in Escherichia coli strain AA7852 carrying a mutation in peptidyl-tRNA hydrolase (Pth) encoded by the pth(Ts) gene. The presence in AA7852 cells of a plasmid bearing lysV gene helped to maintain low levels of the unstable Pth(Ts) protein and to preserve the viability of the mutant line at 41°C whereas plasmids bearing other tRNA genes were ineffective. At 32°C the excess of tRNA(Lys) did not alter the percentages of the free-, charged- or peptidyl-tRNA(Lys) species compared with those found in strains that did not overproduce tRNA(Lys). At 41°C, however, despite increases in the level of peptidyl-tRNA(Lys), the excess tRNA(Lys) helped to maintain the concentration of charged-tRNA(Lys) at a level comparable with that found in non-overproducer cells grown at a permissive temperature. In addition, the excess tRNA(Lys) at 41°C provoked a reduction in the concentrations of various peptidyl-tRNAs, which normally accumulate in pth(Ts) cells, and a proportional increase in the concentrations of the corresponding aminoacyl-tRNAs. The possible mechanism of rescue due to the overexpression of tRNA(Lys) and the causes of tRNA(Lys) starvation in pth(Ts) strains grown at non-permissive temperatures are considered

Protein synthesis factors (RF1, RF2, RF3, RRF, and tmRNA) and peptidyl-tRNA hydrolase rescue stalled ribosomes at sense codons.

During translation, ribosomes stall on mRNA when the aminoacyl-tRNA to be read is not readily available. The stalled ribosomes are deleterious to the cell and should be rescued to maintain its viability. To investigate the contribution of some of the cellular translation factors on ribosome rescuing, we provoked stalling at AGA codons in mutants that affected the factors and then analyzed the accumulation of oligopeptidyl (peptides of up to 6 amino acid residues, oligopep-)-tRNA or polypeptidyl (peptides of more than 300 amino acids in length, polypep-)-tRNA associated with ribosomes. Stalling was achieved by starvation for aminoacyl-tRNA(Arg4) upon induced expression of engineered lacZ (β-galactosidase) reporter gene harboring contiguous AGA codons close to the initiation codon or at internal codon positions together with minigene ATGAGATAA accompanied by reduced peptidyl-tRNA hydrolase (Pth). Our results showed accumulations of peptidyl-tRNA associated with ribosomes in mutants for release factors (RF1, RF2, and RF3), ribosome recycling factor (RRF), Pth, and transfer-messenger RNA (tmRNA), implying that each of these factors cooperate in rescuing stalled ribosomes. The role of these factors in ribosome releasing from the stalled complex may vary depending on the length of the peptide in the peptidyl-tRNA. RF3 and RRF rescue stalled ribosomes by drop-off of peptidyl-tRNA, while RF1, RF2 (in the absence of termination codon), or Pth may rescue by hydrolyzing the associated peptidyl-tRNA. This is followed by the disassembly of the ribosomal complex of tRNA and mRNA by RRF and elongation factor G

Generación de estructuras sociales a partir de dinámicas de movimiento espacial

“En este trabajo se presenta una aplicación del modelo de Ising 2-dimensional que describe segregación de población en términos de interacciones cercanas (con los primeros vecinos) conocida como modelo de segregación de Schelling. A este modelo se le hace una ampliación para considerar términos de riqueza agregándolo como atributo a los agentes y modificando la regla de interacción de Schelling. Se agregó también un término de costos en el terreno donde se mueven los agentes, de manera que se favorezca la segregación en términos del parámetro de riqueza. Con esto se obtuvieron dos patrones de segregación, el primero en términos de la étnia, equivalente a los patrones de segregación de Schelling, el segundo en términos de los parámetros de riqueza, su segregación sólo es significativa para la parte en el terreno donde se asignó un costo. Se hace una propuesta de dinámica de formación de amistad, es decir se propone un algoritmo para la construcción de una red social de amistad asignando atributos de personalidad (qué tipo de personas son), y un atributo de que tan amistosos son los agentes (cuántas amistades puede sostener.