Molecular organisation of an A mating type factor of the basidiomycete fungus Coprinus-cinereus

The Aα3 and Aβ3 genes, which together constitute the A42 mating type factor of Coprinus cinereus, were isolated from a cosmid genomic library by walking 50 kb, a map distance of 0.5 units, from the closely linked metabolic gene pab-1. Cosmid clones having A gene function were identified by transformation into compatible A6 (α2β2) and A5 (α1β1) host cells where either α3 or β3 was expected to elicit the A factor — regulated development of unfused clamp cells. DNAs were digested with various enzymes before transformation in order to identify the smallest fragments containing an active α3 or β3 gene. Two non-overlapping fragments were identified as containing the α3 and β3 genes respectively. Southern hybridisation analyses showed that these two cloned genes had no detectable sequence homology, and that there was little or no hybridisation to the α and β gene alleles that constitute the A5 and A6 factors. α3 and β3 were shown to be less than 2.0 kb apart and embedded in a DNA sequence extending over 9.0 kb which was unique to our A42 strain and may contain a third A factor gene