Functional analysis of SLO2 provides new insight into the role of plant PPR proteins

PPR (Pentatricopeptide repeat) proteins are mainly involved in RNA metabolism. In Arabidopsis, the PPR family is composed of more than 450 members; however, only few of them were functionally characterized. In a previous report,1 we identified a novel mitochondrial PPR RNA editing factor, named SLO2, which is responsible for 7 editing events in Arabidopsis. Loss-of-function mutation in SLO2 results in plant growth retardation, and delayed development, and leads to the dysfunction of mitochondrial complex I, III and IV. slo2 is the first example of a single gene mutation affecting 3 complexes of the mitochondrial electron transport chain. This Short Communication discusses the conservation of upstream regions of editing sites affected by SLO2 and illustrates the effect of mutation of SLO2 on activation of the alternative pathway. We also reflect upon the implications and perspectives of these findings.     

Two modes of binding of DinI to RecA filament provide a new insight into the regulation of SOS response by DinI protein

RecA protein plays a principal role in bacterial SOS response to DNA damage. The induction of the SOS response is well understood and involves the cleavage of the LexA repressor catalyzed by the RecA nucleoprotein filament. In contrast, our understanding of the regulation and termination of the SOS response is much more limited. RecX and DinI are two major regulators of RecA's ability to promote LexA cleavage and strand exchange reaction, and are believed to modulate its activity in ongoing SOS events. DinI's function in the SOS response remains controversial, since its interaction with the RecA filament is concentration dependent and may result in either stabilization or depolymerization of the filament. The 17 C-terminal residues of RecA modulate the interaction between DinI and RecA. We demonstrate that DinI binds to the active RecA filament in two distinct structural modes. In the first mode, DinI binds to the C-terminus of a RecA protomer. In the second mode, DinI resides deeply in the groove of the RecA filament, with its negatively charged C-terminal helix proximal to the L2 loop of RecA. The deletion of the 17 C-terminal residues of RecA favors the second mode of binding. We suggest that the negatively charged C-terminus of RecA prevents DinI from entering the groove and protects the RecA filament from depolymerization. Polymorphic binding of DinI to RecA filaments implies an even more complex role of DinI in the bacterial SOS response.