A light boson U from an extra U (1) interpolates between a generalized dark photon coupled to Q, B and Li (or B − L), plus possibly dark matter, a dark Z coupled to the Z current, and one axially coupled to quarks and leptons. We identify the corresponding U (1)F symmetries, withacting in a dark sector and F ′ on possible semi-inert BEH doublets uncoupled to quarks and leptons. The U current is obtained from the U (1)F and Z currents, with a mixing determined by the spin-0 BEH fields. The charge QU of chiral quarks and leptons is a combination of Q, B, Li and T3L with the axial FA. It involves in general isovector and isoscalar axial terms, in the presence of two BEH doublets.A longitudinal U with axial couplings has enhanced interactions, and behaves much as an axionlike particle. Its axial couplings gA, usually restricted to < ∼ 2 × 10 −7 mU (MeV), lead to effective pseudoscalar ones gP = gA × 2m q,l /mU = 2 1/4 G 1/2 F m q,l A± . A± is proportional to an invisibility parameter r = cos θA induced by a singlet v.e.v., possibly large and allowing the U to be very weakly interacting.This allows for a very small gauge coupling, expressed with two doublets and a singlet as g"/4 ≃ 2 × 10 −6 mU (MeV) r/sin 2β . We discuss phenomenological implications for meson decays, neutrino interactions, atomic-physics parity violation, naturally suppressed π 0 → γ U decays, etc.. The U boson fits within the grand-unification framework, in symbiosis with a SU (4)es electrostrong symmetry broken at the GUT scale, with QU depending on Q, B −L, FA and T3A through three parameters γY , γA and η .