The regulatory unit of strain 168 encompassing the structural genes of the -acetylmuramoyl-L-alanine amidase and of its modifier has been sequenced, and found to be a divergon consisting of divergently transcribed operons and . Proteins LytA, LytB and LytC are endowed with export signal peptides. Mature LytA is a 9.4 kDa, highly acidic polypeptide whose deduced amino acid sequence points to a lipoprotein. LytB and LytC, the modifier and the amidase, are highly basic. After cleavage of the signal sequence their molecular masses are 74.1 and 49.9 kDa, respectively. These two proteins share considerable homology in their N-terminal moieties and have three GSNRY consensus motifs, characteristic of nearly all amidases. The C-terminal moiety of LytB exhibits homology to the product of . LytR is a 35 kDa protein which acts as an attenuator of the expression of both and operons. Transcription of the operon proceeds from two promoters: P, identified as P (Gilman , 1984), and an upstream P. The former only is subject to LytR attenuation. Translational initiation of and is directed by UUG start codons, suggesting that , and undergo coupled translation. Transcription of is initiated at two start sites, one of which corresponds to a highly intense P promoter whereas the other does not seem to share much homology with any of the known promoter consensus sequences. Both promoters are attenuated by LytR. It is confirmed that the synthesis of the amidase is controlled at least in part by SigD, i.e. that it belongs to the regulon and that its activity, or part of it, is co-regulated with flagellar motility. The role of the mutations conferring the Sin, Fla and Ifm phenotypes in the expression of the operon is discussed.


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