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<p>Abstract</p> <p>Open reading frame <it>l0045 </it>in the pathogenic island of enterohemorrhagic <it>Escherichia coli </it>O157:H7 has been predicted to encode a lytic transglycosylase that is homologous to two different gene products encoded by the same bacteria at loci away from the island. To deduce the necessity of the presence in the island, we created an <it>l0045-</it>deleted strain of EHEC and observed that both the level of cytosolic EspA and that of the other type III secreted proteins in the media were affected. In a complementation assay, a low level-expressing L0045 appeared to recover efficiently the type III secretion (TTS). On the other hand, when <it>l0045 </it>was driven to express robustly, the intracellular levels of representative TTS proteins were severely suppressed. This suppression is apparently caused by the protein of L0045 <it>per se </it>since introducing an early translational termination codon abolished the suppression. Intriguingly, the authentic L0045 was hardly detected in all lysates of EHEC differently prepared while the same construct was expectedly expressed in the K-12 strain. A unique network must exist in EHEC to tightly regulate the presence of L0045, and we found that a LEE regulator (GrlA) is critically involved in this regulation.</p>