The Relevance of a Study of a Temperature-sensitive Ballooning Mutant of Aspergillus nidulans Defective in Mannose Metabolism to our Understanding of Mannose as a Wall Component and Carbon/Energy Source

Abstract

SUMMARY: A temperature-sensitive mutant strain of Aspergillus nidulans has been isolated which failed to grow normally on minimal agar medium at 43 °C unless mannose was supplied as a sole carbon source. The mutant has been given the symbol mnr A455 for mannose relief. Under restrictive conditions (43 °C) the mutant produced extensive areas of swollen hyphae, called balloons, and wall preparations from cultures grown at 40 °C had approximately one-third of the mannose found in walls of control cultures. The mnrA455 strain produced a more thermolabile phosphomannose mutase than the wild-type strain, so it is suggested that the mutation is in the structural gene coding for this enzyme. The mnrA455 strain failed to grow on minimal agar medium containing 0·1% (w/v) mannose and 0·9% (w/v) glucose at 43 °C. This property was used to obtain revertants, one of which was a double mutant mnrA455 manAl. Growth of the double mutant in media containing [14C]mannose at 43 °C showed that 84% of the label found in the wall occurred in mannose in comparison with 33% in the wild-type control. Autoradiography of cultures of the double mutant using [3H]mannose showed predominant incorporation of label into the tip region of growing hyphae. A manAl strain, isolated following haploidization of a diploid, was unable to grow on media containing glucose or mannose alone. The manAl strain had reduced phosphomannose isomerase (EC 5.3.1.8) activity. The manAl mutation, which is epistatic to mnrA455, has been located to linkage group VIII and the mnrA455 mutation to linkage group V. A pathway is presented for the utilization of mannose in A. nidulans.