Which letter will replace the question mark (?) in the following series?

S31T, U37V, W41X, ?

Y43Z

According to the above alphabet position:

S31T    ⇒ (19), 31, (20)

U37V   ⇒ (21), 37, (22)

W41X  ⇒ (23), 41, (24)

?         ⇒ (25), 43, (26) = Y43Z

(consecutive alphabet series at 1st and last alphabet position and there is prime number series in the middle)