Enhanced function conferred on low-abundance chemoreceptor Trg by a methyltransferase-docking site

In Escherichia coil, 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 met hyl-accepting activity, in establishing a functional balance between the tw o directions of flagellar rotation, in timely adaptation, and in mediating efficient chemotaxis. Low-abundance receptors are not, even when their cell ular content is increased. We found that the low-abundance receptor Trg acq uired essential functional features of a high-abundance receptor by the add ition of the final 19 residues of the high-abundance receptor Tsr. The carb oxy terminus of this addition carried a methyltransferase-binding pentapept ide, NWETF, present in high-abundance receptors but absent in the low-abund ance class. Provision of this docking site not only enhanced steady-state a nd adaptational methylation but also shifted the abnormal, counterclockwise bias of flagellar rotation toward a more normal rotational balance and vas tly improved chemotaxis in spatial gradients. These improvements can be und erstood as the result of both enhanced kinase activation by the more methyl ated receptor and timely adaptation by more efficient methyl-accepting acti vity. We conclude that the crucial functional difference between the low-ab undance receptor Trg and its high-abundance counterparts is the level of me thyl-accepting activity conferred by the methyltransferase-docking site.