Each letter of the first group is replaced by the two letters one come after it and one that comes before it in the second group.

The series is yxxy/yxxy/yxxy/yxxy. Thus, the pattern ‘yxxy’ is repeated.

BY, CX, EV, GT, KP, __

Explanation:

R^-7, K^-5, F^-3, C, __

G⁺³, J⁺⁴, N⁺³, Q⁺⁴, U⁺³, __