77. Smooth movement of bacteria during chemotaxis is due to
    (1) Due to tumbling
    (2) Phosphorylation of Chey
    (3) Movement of H⁺ across plasma membrane
    (4) Phosphorylation of Che A

Smooth Movement in Bacterial Chemotaxis: The Role of Flagellar Rotation

Understanding Bacterial Chemotaxis

Bacteria move toward attractants or away from repellents through a process called chemotaxis. This movement alternates between two behaviors:

• Running: Smooth swimming in a roughly straight line.

• Tumbling: A brief reorientation that changes the swimming direction.

What Causes Smooth Movement?

Smooth movement, or “running,” is primarily due to the counterclockwise (CCW) rotation of flagella. When the flagella rotate CCW, they form a cohesive bundle that propels the bacterium forward smoothly.

In contrast, clockwise (CW) rotation causes the flagellar bundle to disassemble, leading to tumbling and random reorientation.

Molecular Control

The swimming and tumbling behaviors are regulated by a set of proteins in the chemotaxis signaling pathway:

• CheA: A histidine kinase that autophosphorylates upon receptor activation.

• CheY: The response regulator that, when phosphorylated (CheY-P), interacts with the flagellar motor to induce CW rotation (tumbling).

When CheY is unphosphorylated, flagella rotate CCW, resulting in smooth running.

Evaluating the Answer Choices

1. Due to tumbling: Incorrect, because tumbling causes random reorientation, not smooth movement.

2. Phosphorylation of CheY: Incorrect for smooth movement; phosphorylation of CheY induces tumbling, not running.

3. Movement of H+ across plasma membrane: Proton motive force powers flagellar rotation but is not directly the reason for smooth versus tumbling movement.

4. Phosphorylation of CheA: Part of the signaling cascade but does not directly cause smooth movement; it phosphorylates CheY, which regulates rotation direction.

Correct Answer

Smooth movement during bacterial chemotaxis is due to counterclockwise rotation of flagella caused by the absence of phosphorylated CheY, hence it is not due to tumbling or CheY phosphorylation directly.

The best fit from given options is:

(2) Phosphorylation of CheY is linked to switching movement behavior, but since phosphorylation of CheY causes tumbling, it is not responsible for smooth movement.

Therefore, none explicitly state the mechanism behind smooth movement but based on the choices, phosphorylation of CheY is closely related to changes controlling movement.

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Summary:

• Smooth movement = CCW rotation, unphosphorylated CheY.

• Tumbling = CW rotation, phosphorylated CheY.