  We analytically compute the two-loop scattering amplitude $gg \to Hg$
assuming that the mass of the quark, that mediates the ggH interaction, is
vanishingly small. Our computation provides an important ingredient required to
improve the theoretical description of the top-bottom interference effect in
Higgs boson production in gluon fusion, and to elucidate its impact on the
Higgs boson transverse momentum distribution.
