Degradation of cellulose
In nature, the oyster mushroom grows on dead trunks of trees where the lignin and cellulose - the two principal components of wood - are being broken down. Degradation of lignin has been studied over the years by a number of research teams that have characterised the genes involved. But nobody, to date, has tackled the degradation of cellulose from a molecular perspective.
Cellulose is the most abundant biological polymer on the planet. It is made up of units of D-glucose united by means of glycosidic links that form long polymer chains. The breaking down by live organisms takes place through the action of three types of enzymes: endogluconases, cellobiohydrolases y b-glucosidases.
All these, necessary for the complete breaking down of cellulose, function by hydrolysing the glycoside links, but they vary in the specificity of substrate: the endoglucanases attack the glycosidic links within the cellulose molecule, the cellobiohydrolases act by liberating units of cellobiose from either end of the cellulose chain and the b-glucosidases hydrolyse the cellobiose molecules, producing glucose as end product.
In her PhD thesis Arantza Eizmendi Goikoetxea has analysed the activity of one of these types of enzymes: the cellobiohydrolases. To this end, she cloned, isolated and sequenced those genes of the oyster mushroom responsible for this activity and investigated the culture in which each of these genes expresses itself.
Five genes of one family
The PhD work resulted in the isolation of five genes of the oyster mushroom, of the Florida variety, and the expression thereof giving rise to different cellobiohydrolases, thus demonstrating the existence of a multigenic family responsible for the said enzymatic activity. Also, using such genomic sequences as a probe, it has been possible to detect what are the conditions under which the expression of each one of the genes is produced. This has enabled the synthesis of the cDNA of each gene and, by means of comparison with the corresponding genomic sequence, the characterisation of their structure.
Regarding their location on the linkage map, it has been found that four of the five genes are located on the same chromosome, quite near each other, and the other is located on a different chromosome. It is precisely this fifth gene that is structurally distinct from the others: it lacks a fragment at its end.
It should be pointed out that the genes that are together and the lone one are located on chromosomes where there are also genes responsible for the breaking down of lignin. This is of great interest because lignin and cellulose are found together in nature. They are found together in wood and it would seem logical that the genes responsible for the degradation of one or the other are located close together on the genome, on the same chromosomes.