Enhanced Function Conferred on Low-Abundance Chemoreceptor Trg by a Methyltransferase-Docking Site

In Escherichia coli, high-abundance chemoreceptors are present in cellular amounts approximately 10-fold higher than those of low-abundance receptors. These two classes exhibit inherent differences in functional activity. As sole cellular chemoreceptors, high-abundance receptors are effective in methyl-accepting activity, in establishing a functional balance between the two directions of flagellar rotation, in timely adaptation, and in mediating efficient chemotaxis. Low-abundance receptors are not, even when their cellular content is increased. We found that the low-abundance receptor Trg acquired essential functional features of a high-abundance receptor by the addition of the final 19 residues of the high-abundance receptor Tsr. The carboxy terminus of this addition carried a methyltransferase-binding pentapeptide, NWETF, present in high-abundance receptors but absent in the low-abundance class. Provision of this docking site not only enhanced steady-state and adaptational methylation but also shifted the abnormal, counterclockwise bias of flagellar rotation toward a more normal rotational balance and vastly improved chemotaxis in spatial gradients. These improvements can be understood as the result of both enhanced kinase activation by the more methylated receptor and timely adaptation by more efficient methyl-accepting activity. We conclude that the crucial functional difference between the low-abundance receptor Trg and its high-abundance counterparts is the level of methyl-accepting activity conferred by the methyltransferase-docking site.