The bacterial parD toxin-antitoxin system of plasmid R1 encodes two proteins, the Kid toxin and its cognate
antitoxin, Kis. Kid cleaves RNA and inhibits protein synthesis and cell growth in Escherichia coli. Here, we
show that Kid promotes RNA degradation and inhibition of protein synthesis in rabbit reticulocyte lysates.
These new activities of the Kid toxin were counteracted by the Kis antitoxin and were not displayed by the
KidR85W variant, which is nontoxic in E. coli. Moreover, while Kid cleaved single- and double-stranded RNA
with a preference for UAA or UAC triplets, KidR85W maintained this sequence preference but hardly cleaved
double-stranded RNA. Kid was formerly shown to inhibit DNA replication of the ColE1 plasmid. Here we
provide in vitro evidence that Kid cleaves the ColE1 RNA II primer, which is required for the initiation of
ColE1 replication. In contrast, KidR85W did not affect the stability of RNA II, nor did it inhibit the in vitro
replication of ColE1. Thus, the endoribonuclease and the cytotoxic and DNA replication-inhibitory activities of
Kid seem tightly correlated. We propose that the spectrum of action of this toxin extends beyond the sole
inhibition of protein synthesis to control a broad range of RNA-regulated cellular processes.
This work was supported by grants from the Spanish Ministerio de
Ciencia y Tecnología (SAF-2002-04649), the CSIC (PIES 200420E006
and 200420E062), and the Spanish REIPI Network of the “Fondo de
Investigaciones Sanitarias” (CO314).
Peer reviewed