How to pull low with a transistor?
I assume your "3.3V" line is tied to 3.3V through some sort of resistance, and that ground is the rail you are trying to pull it to when MCU outp to the base is high.  Swap connections: Your high impedance normally 3.3V line should be tied to the NPN's collector, and the ground rail tied to the NPN's emitter.  With that done, a +5V signal from the MCU to the base of your NPN (use an in-line current limiting resitor to the base!) will pull the collector low.