FILAMENTOUS PHAGE IKE MESSENGER-RNAS CONSERVE FORM AND FUNCTION DESPITE DIVERGENCE IN REGULATORY ELEMENTS

As a means of determining whether there has been selection to conserve the basic pattern of filamentous phage mRNAs, the major mRNAs represe nting genes II to VIII have been defined for a phage distantly related to the Ff group specific for Escherichia coli hosts bearing F pili. P hage IKe has a genome with 55% identity with the Ff genome and infects E. coli strains bearing N pili. The results reveal a remarkably simil ar pattern of overlapping polycistronic mRNAs with a common 3' end and unique 5' ends. The IKe mRNAs, like the Ff phage mRNAs, represent a c ombination of primary transcripts and processed RNAs. However, examina tion of the sequences containing the RNA endpoint positions revealed t hat effectively the only highly conserved regulatory element is the rh o-independent terminator that generates the common 3' end. Promoters a nd processing sites have not been maintained in identical positions, b ut frequently are placed so as to yield RNAs with similar coding funct ion By conserving the pattern of transcription and processing despite divergence in the regulatory elements and possibly the requirements fo r host endoribonucleases, the results argue that the pattern is not si mply fortuitous. (C) 1997 Academic Press Limited.