Nonsense polarity in most cases depends onactivation of cryptic transcription terminators. Wefound that the strong polar effect observed in thenonsense polar mutant R4 of phage f1, mapping inthe 5’ proximal region of gene III, instead dependson enhanced instability of mutant mRNAs, whosepattern can be restored by reduction of RNase Eactivity. rne -(ts) E. coli strains allowed to explorethe mechanisms underlying f1 mRNA processingand degradation.The major gene III species, a 1.8 Kb long molecule,appeared to be a secondary transcript, whose decayis modulated by a REP, located at its 3' end. TheRNA pool of a mutagenized phage unable to formthat structure, lacks completely that transcript.Infection of RNaseE-(ts) cells resulted in theaccumulation of very large transcripts (3.9-6.4 Kb)and the reduction of the level of the 1.8 Kbtranscript. The pool of gene III messages found atnon permissive temperature includes transcriptscomposed uniquely by coding sequences (presentalso in RNase E+ cells) and mRNAs containing, intheir internal portion, the untranslated sequences ofthe IG region. Thus it appears that in wild type cellsthese sequences are transcribed and promptly cutby RNase E. Cloning of the IG sequences togetherwith last nt of gene IV into a plasmid, maintainingthe natural translation frame, confirmed thepresence of RNase E cutting sites into the IG,whose translation in suppressor cells makes themresistant to this ribonuclease. Thus ribosomeactivity appears to challenge that of the RNase E inmRNA processing.
|Stato di pubblicazione||Published - 2011|