AN EVOLUTIONARY LINK BETWEEN SPORULATION AND PROPHAGE INDUCTION IN THE STRUCTURE OF A REPRESSOR - ANTI-REPRESSOR COMPLEX

Spore formation is an extreme response of some bacteria to adversity. Ln Bacillus subtilis the proteins of the sin, sporulation inhibition, region form a component of an elaborate molecular circuitry that regul ates the commitment to sporulation. SinR is a tetrameric repressor pro tein that binds to the promoters of genes essential for entry into spo rulation and prevents their transcription. This repression is overcome through the activity of SinI., which disrupts the SinR tetramer throu gh the formation of a SinI-SinR heterodimer. The interactions governin g this curious quaternary transition are revealed in the crystal struc ture of the SinI-SinR complex. The most striking, and unexpected, find ing is that the tertiary structure of the DNA-binding domain of SinR i s identical with that of the corresponding domains of the repressor pr oteins, CI and Cro, of bacteriophage 434 that regulate lysis/lysogeny. This structural similarity greatly exceeds that between SinR and any bacterial protein or between the 434 repressor proteins and their homo logues in the closely related bacteriophage lambda. The close evolutio nary relationship implied by the structures of SinR and the 434 repres sors provokes both comparison of their functions and a speculative con sideration of the intriguing possibility of an evolutionary link betwe en the two adaptive responses, sporulation and prophage induction. (C) 1998 Academic Press.