MECHANISM OF POST-SEGREGATIONAL KILLING - TRANSLATION OF HOK, SRNB AND PND MESSENGER-RNAS OF PLASMIDS R1, F AND R483 IS ACTIVATED BY 3'-ENDPROCESSING

The gene systems hok/sok of R1, srnB of F and pnd of R483 mediate plas mid maintenance by killing of plasmid-free segregants. Translation of the very stable mRNAs encoding the killer proteins is regulated by sma ll unstable antisense RNAs. The differential decay rates of the inhibi tory antisense RNAs and the mRNAs encoding the killer proteins is the basis for the onset of killer mRNA translation in newborn plasmid-free segregants and the killing of these cells. We have suggested previous ly that this requires that the killer mRNAs occur in two forms. A tran slationally inactive form was proposed to be converted into a 3'-trunc ated, translationally active mRNA. In the presence of the antisense RN A, translation from this killer mRNA should be inhibited. In this comm unication we present in vivo and in vitro evidence that support this m odel. The requirement for 3'-processing for killer gene expression is demonstrated. By using in vitro techniques it is shown that full-lengt h Hok mRNA is translationally inactive, whereas a 3'-end truncated ver sion of the Hok mRNA is translationally active. In vitro secondary str ucture probing suggests that the 3'-end of the full-length Hok mRNA fo lds back onto the translational initiation region of the mok gene and thereby inhibits translation of the mRNA. By inference we conclude tha t the Pnd and SrnB mRNAs are regulated by a similar mechanism.