Catalyzes the reduction of a single double bond in the isoprenoid tail of menaquinone (MK-9) in M.smegmatis, likely the beta-isoprene unit, forming the predominant form of menaquinone found in mycobacteria, MK-9(II-H2). AH2 + menaquinone-9 = A + beta-dihydromenaquinone-9 Quinol/quinone metabolism; menaquinone biosynthesis. Cells lacking this gene show complete abolition of the synthesis of MK-9(II-H2) accompanied by accumulation of MK-9. They have similar growth rates than wild-type in both aerated and hypoxic culture conditions. However, mycobacterial electron transport efficiency is reduced by 3-fold in mutant cells, but mycobacteria are able to maintain ATP levels by increasing the levels of the total menaquinone in the membrane. Proton motive force (PMF) and oxidative phosphorylation are not affected. Belongs to the geranylgeranyl reductase family.