Is interfacing the 5V TTL UART with 3.3V microcontroller via voltage divider possible?
This is quite commonly done but keep resistances low so that there is little chance the rise times and fall times of the data signal are not unduly lengthened. As PeterJ says in his comment, something around 1k will be OK.

There are devices that you can buy that do this (8 bit wide and bi-directional) but if you only have one signal to modify then resistors are the best choice.