Bifunctional enzyme that catalyzes the GTP-dependent successive addition of multiple gamma-linked L-glutamates to the L-lactyl phosphodiester of 7,8-didemethyl-8-hydroxy-5-deazariboflavin (F420-0) to form polyglutamated F420 derivatives, and the FMNH2-dependent reduction of dehydro-F420-0 to form F420-0. GTP + L-glutamate + oxidized coenzyme F420-0 = GDP + H(+) + oxidized coenzyme F420-1 + phosphate GTP + L-glutamate + oxidized coenzyme F420-1 = GDP + H(+) + oxidized coenzyme F420-2 + phosphate GTP + L-glutamate + oxidized coenzyme F420-(gamma-L-Glu)(n) = GDP + H(+) + oxidized coenzyme F420-(gamma-L-Glu)(n+1) + phosphate FMN + H(+) + oxidized coenzyme F420-0 = dehydro coenzyme F420-0 + FMNH2 Binds 2 divalent metal cations per subunit. The ions could be magnesium and/or manganese. Monovalent cation. The ion could be potassium. Optimum pH is 8.5. Cofactor biosynthesis; coenzyme F420 biosynthesis. In the N-terminal section; belongs to the CofE family.