New to Arduino with a simple problem.

Presently, I am controlling the circuit in the drawing with an input to an NPN transistor from the Arduino. Is there a way to control this circuit with just Arduino inputs and outputs? The motor controller is under its own power and the +5 V signal is just that, a signal with no substantial load.

That all depends on the motor controller - what current does it take for instance? Has it opto-isolator
inputs? Datasheet?

No data sheet available. I received it as an on/off controller with a mechanical switch. The signal is 4.9 V at about .14mA. I have replaced the switch with a transistor where the base is controlled by an arduino output. As mentioned, I'd like to eliminate the transistor altogether and use just the arduino.

Sounds possible from your description of 4.9V, 0.14mA when pulled low.
Arduino pin can do that.
Make sure grounds between devices are connected.

Thanks Crossroads-

Worked just fine.