EXCISION OF A P4-LIKE CRYPTIC PROPHAGE LEADS TO ALP PROTEASE EXPRESSION IN ESCHERICHIA-COLI

The Escherichia coli K-12 alpA gene product, when overproduced from a multicopy plasmid, leads to suppression of the capsule overproduction and UV sensitivity phenotypes of cells mutant for the Lon ATP-dependen t protease. This suppression has previously been shown to correlate wi th increased in vivo activity of a previously unknown energy-dependent proteolytic activity capable of degrading Lon substrates, the Alp pro tease. We show in an accompanying paper that alpA, which has homology to a short open reading frame in bacteriophage P4, acts as a positive transcriptional regulator of slpA, a gene linked to alpA and necessary for suppression of lon mutants (J. E. Trempy, J. E. Kirby, and S. Got tesman, J. Bacteriol. 176:2061-2067). The sequence of slpA suggests th at it encodes an integrase gene closely related to P4 int and that bot h alpA and slpA are part of a cryptic P4-like prophage. AlpA expressio n increases SlpA synthesis. Increased SlpA leads, in turn, to the exci sion and loss of the cryptic prophage. Excision is dependent on integr ation host factor as well as on SlpA. Prophage excision is necessary b ut not sufficient for full expression of the Alp protease. A second fu nction (named AHA) allows full protease expression; this function can be provided by the kanamycin resistance element from Tn903 when the el ement is present on a multicopy plasmid. Excision and loss of the cryp tic prophage apparently allow expression of the Alp protease by inacti vating a small stable RNA (10Sa RNA) encoded by the ssrA gene. The pre cursor of this RNA has its 3' end within the cryptic prophage; the mat ure 3' end lies within the prophage attL site. Inactivation of ssrA by insertional mutagenesis is sufficient to allow expression of the supp ressing Alp protease, even in the presence of the cryptic prophage. Th erefore, 10Sa RNA acts as a negative regulator of protease synthesis o r activity, and prophage excision must inactivate this inhibitory func tion of the RNA.