Ans.
The Che proteins (chemotaxis proteins) are cytoplasmic proteins that play an important role in signal transduction pathways between the repellent or attractant and flagellar motor switch of a bacterium. Bacterial cell membranes contain different chemoreceptors for different attractants and repellents.
When a repellent or attractant binds with a chemoreceptor, stimulation of signal-transduction pathway occurs. It sends a signal to the cytoplasmic domain of receptor, which interacts with Che proteins, present at cytoplasm. The rate of auto-phosphorylation decreases when an attractant binds with receptor, while the rate of auto-phosphorylation increases when an attractant binds with receptor.
Thus, 'the attractant enhances smooth swimming and counterclockwise rotation towards the chemical source, which repellent enhances tumbling clockwise rotation away from a chemical source.'