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The tomato resistance protein Sw-5b differs from the classical coiled-coil nucleotide-binding leucine-rich repeat ( CC- NB- LRR) resistance proteins by having an extra N-terminal domain ( NTD). To understand how NTD, CC and NB- LRR regulate autoinhibition and activation of Sw-5b, we dissected the function(s) of each domain., When viral elicitor was absent, Sw-5b LRR suppressed the central NB- ARC to maintain autoinhibition of the NB- LRR segment. The CC and NTD domains independently and additively enhanced the autoinhibition of NB- LRR., When viral elicitor was present, the NB- LRR segment of Sw-5b was specifically activated to trigger a hypersensitive response. Surprisingly, Sw-5b CC suppressed the activation of NB- LRR, whereas the extra NTD of Sw-5b became a positive regulator and fully activated the resistance protein, probably by relieving the inhibitory effects of the CC. In infection assays of transgenic plants, the NB- LRR segment alone was insufficient to confer resistance against Tomato spotted wilt tospovirus; the layers of NTD and CC regulation on NB- LRR were required for Sw-5b to confer resistance., Based on these findings, we propose that, to counter the negative regulation of the CC on NB- LRR, Sw-5b evolved an extra NTD to coordinate with the CC, thus developing a multilayered regulatory mechanism to control autoinhibition and activation. [ABSTRACT FROM AUTHOR]