The Arabidopsis thaliana F-Box Protein FBL17 Is Essential for Progression through the Second Mitosis during Pollen Development

In fungi and metazoans, the SCF-type Ubiquitin protein ligases (E3s) play a critical role in cell cycle regulation by degrading negative regulators, such as cell cycle-dependent kinase inhibitors (CKIs) at the G1-to-S-phase checkpoint. Here we report that FBL17, an Arabidopsis thaliana F-box protein, is involved in cell cycle regulation during male gametogenesis. FBL17 expression is strongly enhanced in plants co-expressing E2Fa and DPa, transcription factors that promote S-phase entry. FBL17 loss-of-function mutants fail to undergo pollen mitosis II, which generates the two sperm cells in mature A. thaliana pollen. Nonetheless, the single sperm cell-like cell in fbl17 mutants is functional but will exclusively fertilize the egg cell of the female gametophyte, giving rise to an embryo that will later abort, most likely due to the lack of functional endosperm. Seed abortion can, however, be overcome by mutations in FIE, a component of the Polycomb group complex, overall resembling loss-of-function mutations in the A. thaliana cyclin-dependent kinase CDKA;1. Finally we identified ASK11, as an SKP1-like partner protein of FBL17 and discuss a possible mechanism how SCFFBL17 may regulate cell division during male gametogenesis

Transferencia interespecífica del factor episómico que condiciona la segregación de glutamato en Citrobacter intermedium C3

[EN] C. intermedium C3 is a bacterial strain which has a extrachromosomal genetic factor (S factor) that in his integrated state produces a failure on the regulation of amino acid synthesis, by showing extracellular glutamate accumulation (2) (3) (8). The episomal nature of S factor is principally based on the studies of transfer within the same C. intermedium C3 and on the effects of O.A. and agitation (4) (9). The present work shows the experiments that have made posible to transfer S factor from C. intermedium C3 to Paracolobactrum intermedium (ATCC 1166) which had never shown glutamate segregation.[ES] Se intenta ver la importancia que pueda tener la selección dependiente de las frecuencias génicas como posible mecanismo responsable de la conservación de la variabilidad en las poblaciones naturales. Se estudia la viabilidad relativa de cinco mutantes de Drosophila melanogaster, cada uno de ellos en competencia con normal, en tres proporciones genotípicas y a dos niveles de densidad de población. A densidad de población baja se encuentra que de los cinco casos estudiados, en cuatro de ellos la viabilidad del genotipo mutante tiende a aumentar cuando éste se encuentra en proporción baja y a disminuir cuando su proporción es elevada. Si la densidad de población es elevada no se observa esta relación. Estos resultados sugieren que la selección dependiente de las frecuencias génicas puede ser importante en poblaciones cuyo número de individuos esté regulado por factores independientes de la densidad en el sentido de Andrewartha y Birch.Peer reviewe