The complete nucleotide sequence of the lysR gene, which encodes the activatory protein required for lysA expression, has been determined. Bal31 deletions and translational fusions were used to localize the promoter region and the initiator ATG of the lysR gene which encodes a 311 amino acid polypeptide. Both lysA and lysR coding sequences were found to be divergent and separated by a very short intergenic region consisting of 121 base-pairs between the postulated ATGs of the two proteins. Transfer of the whole lysR gene on a plasmid carrying a lysR-lacZ fusion shows that lysR expression is autoregulated by a factor of 7. The same binding site (73 base-pairs fragment) could be involved in both effects of the LysR product, acting simultaneously as an operator for lysR expression and an initiator for lysA expression. The genetic organization of the whole region (4127 base-pairs) is given. A strikingly symmetrical pattern is observed with the four tightly packed galR, lysA, lysR and orfX (an unidentified open reading frame) genes, in a very unusual arrangement of both divergent and convergent overlapping transcription units.