F ibrosis is associated with many liver diseases, including hepatitis C virus infection, iron deposition, alcohol consumption, and nonalcoholic fatty liver disease. Hepatic fibrosis results from a net increased synthesis and decreased degradation of extracellular matrix (ECM) proteins. Type I collagen is the most prevalent ECM protein deposited, 1 with activated hepatic stellate cells (HSCs) serving as the primary source. Following a fibrogenic stimulus, HSCs activate from their normal quiescent state, whereby they increase synthesis of procollagen type I messenger RNA (mRNA) and protein, 1,2 and increase cellular proliferation, migration, and contractility. 3,4 Excess ECM accumulation results in scarring within the tissue. Our understanding of ECM degradation during hepatic fibrosis is still very limited. ECM degradation is mediated by matrix metalloproteinases (MMPs), a family of zinc-dependent enzymes grouped into collagenases, gelatinases, stromelysins, and membrane-type MMPs, 5 based upon their substrates. Interstitial collagenases (MMP-1 and MMP-13 in humans,

Aerobic gram-negative methylotrophs oxidize methanol to formaldehyde by using a methanol dehydrogenase that has pyrroloquinoline quinone (PQQ) as a prosthetic group. Seventy-two mutants which are unable to grow on methanol unless the growth medium is supplemented with PQQ have been isolated in the facultative methanol utilizer Methylobacterium extorquens AM1. In addition, 12 previously isolated methanol oxidation mutants of M. extorquens AM1 were shown to be able to grow on methanol in the presence of PQQ. These putative PQQ biosynthesis mutants have been complemented by using previously isolated clones containing M. extorquens AM1 DNA, which were known to contain genes necessary for oxidation of methanol to formaldehyde (mox genes (27). MDH is a tetrameric enzyme located in the periplasm that contains pyrroloquinoline quinone (PQQ) as the prosthetic group and also contains Ca2+ (37,40). PQQ was first identified as the prosthetic group of MDH and is now also known to be the prosthetic group of a few other bacterial dehydrogenases that oxidize alcohols or sugars (9). The biosynthetic pathway of PQQ has not yet been determined, but the biosynthetic precursors are known to be tyrosine and glutamate (19).

Methanol dehydrogenase, the enzyme that oxidizes methanol to formaldehyde in gram-negative methylotrophs, contains the prosthetic group pyrroloquinoline quinone (PQQ). To begin to analyze how the synthesis of PQQ is coordinated with the production of other methanol dehydrogenase components, the transcription of one of the key PQQ synthesis genes has been studied. This gene (pqqD) encodes a 29-amino-acid peptide that is thought to be the precursor for PQQ biosynthesis. A unique transcription start site was mapped to a guanidine nucleotide 95 bp upstream of the pqqD initiator codon. RNA blot analysis identified two transcripts, a major one of 240 bases encoding pqqD and a minor one of 1,300 bases encoding pqqD and the gene immediately downstream, pqqG. Both transcripts are present at similar levels in cells grown on methanol and on succinate, but the levels of PQQ are about fivefold higher in cells grown on methanol than in cells grown on succinate. These results suggest that PQQ production is regulated at a level different from the transcription of pqqD. The genes mxbM, mxbD, mxcQ, mxcE, and mxaB are required for transcription of the genes encoding the methanol dehydrogenase subunits and were assessed for their role in PQQ production. PQQ levels were measured in mutants defective in each of these regulatory genes and compared with levels of pqqD transcription, measured with a transcriptional fusion between the pqqD promoter and xylE. The results showed that only a subset of these regulatory genes (mxbM, mxbD, and mxaB) is required for transcription of pqqD, and only mxbM and mxbD mutants affected the final levels of PQQ significantly.Methylobacterium extorquens AM1 is a facultative methylotrophic bacterium capable of growing on single-carbon compounds such as methanol and methylamine as well as multicarbon compounds such as succinate (13). Methanol is oxidized to formaldehyde by the periplasmic enzyme methanol dehydrogenase, and then the formaldehyde is either assimilated into the cell or is oxidized to CO 2 , with the generation of energy. The methanol dehydrogenase contains pyrroloquinoline quinone (PQQ) as the prosthetic group (2).A complex array of genes is involved in methanol oxidation (14, 15) in M. extorquens AM1, and functions have been determined for a number of them. mxaF encodes the large subunit of methanol dehydrogenase, mxaI encodes the methanol dehydrogenase small subunit, and mxaG encodes the cytochrome c L structural polypeptide (1,22,23). mxaA, mxaK, and mxaL are involved in the insertion of calcium into the active site of methanol dehydrogenase (22-24). mxbM, mxbD, mxcQ, mxcE, and mxaB are genes required for transcription of methanol oxidation genes (14,20,22,23,29).The transcriptional regulation of the mxaF promoter in M. extorquens AM1 regulatory mutants has been studied, and in the wild-type strain, a sixfold increase in mxaF transcription was found in cells grown on methanol compared with cells grown on succinate. In strains defective in mxcQ, mxcE, mxbM, mxbD, or mxaB, the transcription f...

Methylobacterium extorquens AM1 pqqEF are genes required for synthesis of pyrroloquinoline quinone (PQQ). The nucleotide sequence of these genes indicates PqqE belongs to an endopeptidase family, including PqqF of Klebsiella pneumoniae, and M. extorquens AM1 PqqF has low identity with the same endopeptidase family. M. extorquens AM1 pqqE complemented a K. pneumoniae pqqF mutant.

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