Bifunctional enzyme that catalyzes the GTP-dependent successive addition of two or more 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 FMN + H(+) + oxidized coenzyme F420-0 = dehydro coenzyme F420-0 + FMNH2 GTP + L-glutamate + oxidized coenzyme F420-1 = GDP + H(+) + oxidized coenzyme F420-2 + phosphate Binds 2 divalent metal cations per subunit. The ions could be magnesium and/or manganese. Monovalent cation. The ion could be potassium. Cofactor biosynthesis; coenzyme F420 biosynthesis. In the N-terminal section; belongs to the CofE family. Was originally (PubMed:15215601) thought to be a probable tetracycline dehydrogenase since it is essential to the final reaction in the biosynthesis of 6-demethyltetracycline, but enzymatic activity has not been proven. Several facts suggest it is more likely to be a coenzyme F420:L-glutamate ligase: it is a homolog of the characterized protein CofE from Methanococcus jannaschii and FbiB from Mycobacterium smegmatis which are involved in the polyglutamylation of F420-0; F420 is used for tetracycline biosynthesis, probably being necessary for the last step; the gene coding for this protein is close to a homolog of the characterized protein CofD from Methanococcus jannaschii involved in F420-0 production.