Co-expression of the mating-type genes involved in internuclear recognition is lethal in Podospora anserina

In the heterothallic filamentous fungus Podospora anserina, four mating-typ e genes encoding transcriptional factors have been characterized: FPR1 in t he mat+ sequence and FMR1, SMR1, and SMR2 in the alternative mat- sequence. Fertilization is controlled by FPR1 and FMR1. After fertilization, male an d female nuclei, which have divided in the same cell, form mat+/mat- pairs during migration into the ascogenous hyphae. Previous data indicate that th e formation of mat+ / mat- pairs is controlled by FPR1, FMR1, and SMR2. SMR 1 was postulated to be necessary for initial development of ascogenous hyph ae. In this study, we investigated the transcriptional control of the mat g enes by seeking mat transcripts during the vegetative and sexual phase and fusing their promoter to a reporter gene. The data indicate that FMR1 and F PR1 are expressed in both mycelia and perithecia, whereas SMR1 and SMR2 are transcribed in perithecia. Increased or induced vegetative expression of t he four mat genes has no effect when the recombined gene is solely in the w ild-type strain. However, the combination of resident FPR1 with deregulated SMR2 and overexpressed FMR1 in the same nucleus is lethal. This lethality is suppressed by the expression of SMR1, confirming that SMR1 operates down stream of the other mat genes.