The liberation of carbon from polymers such as fun gal cell wall carbohydrates and secreted proteins is indicated by increased expression of glycosyl hydro lases and proteases as well as by increased extracel lular protease activity. Strikingly, the major secreted protease PepA was the second most abundant extracellular protein during carbon starvation, which was only excelled directly by protein levels of the maltose induced alpha glucosidae GlaA secreted during exponential growth. Although transcripts of the ChiB NagA chitinolytic system accumulated simultane ously during carbon starvation as described previously for A. nidulans, only NagA could be identi?ed extra cellularly in high relative abundances. While the low relative abundance of ChiB in ?ltrates from day 1 is in agreement with the absence of a predicted signal peptide sequence, it con?icts with results obtained in A.
nidulans, where it was identi?ed as the major extracellular autolytic chitinase. Interestingly, despite its extracellular abundance, also A. nidulans ChiB lacks a signal peptide prediction. Whether A. nidulans ChiB is released by non classical secretion Inhibitors,Modulators,Libraries or lysis remains to be shown. It is tempting to speculate that cell wall degrading hydrolases lacking a signal peptide sequence are part of the fungal PCD program and accumu late intracellularly in dying compartments to be subse quently released upon cell death for recycling of the remaining hyphal ghost. In view of the natural emerse growth of fungi, this could be a successful strategy for survival released hydrolases will remain localized to hyphal ghosts and not become diluted as under sub merged conditions.
Future studies will be necessary to elucidate whether intracellular localization, retention at the cell wall, Inhibitors,Modulators,Libraries protein instability or ine?cient transla tion Inhibitors,Modulators,Libraries explain the low abundance of ChiB in ?ltrates of A. niger. Carbon starvation provoked asexual reproduction of A. niger, which was clearly evident by the formation of condiospores and by expression of respec tive conidiation related genes. This elaborate developmental program requires liberation and recy cling of carbon to proceed in aging batch cultures. Increased heterogeneity and compartmental ization of the hyphal network resulting in empty, cryp tically growing and conidiating compartments implies an ordered form of fungal cell death ensuring self propagation to survive life threatening starvation condi tions.
Inhibitors,Modulators,Libraries In A. nidulans it was shown that disruption of the ?bA gene, encoding a regulator of G protein signal ing acting upstream of BrlA, resulted in an enhanced autolytic phenotype. Hence, vegetative growth, autol ysis and conidiation Inhibitors,Modulators,Libraries are closely interwoven processes and future factorial sellekchem genome wide transcriptomic studies of wild type and developmental mutants will allow decon struction of fungal cell death and its link to developmental processes.