“Mozart’s last three symphonies come from the extraordinarily creative summer of 1788,” (RedlandsSymphony.com) “In the space of slightly over six weeks, he composed the Symphony in E-flat, K. 543; the Symphony in G minor, K. 550; and the Symphony in C major, K. 551 …
Of the three 1788 symphonies, the Symphony in G minor, K. 550 (popularly referred to as No. 40, but probably No. 53), is the most original and has had the greatest influence on future composers. Few works from the 18th century are as intense, chromatic, and unconventional … few classical works more clearly point the way toward 19th century romanticism.”
Beginning in G minor, the movement transitions to the relative major key, Bb, for the first time at 0:27. Many other shifts in tonality follow.