FIL1, A G-PROTEIN ALPHA-SUBUNIT THAT ACTS UPSTREAM OF CAMP AND IS ESSENTIAL FOR DIMORPHIC SWITCHING IN HAPLOID CELLS OF USTILAGO HORDEI

A constitutive mutation, fill, that causes filamentous growth in the h aplophase of the dimorphic smut fungus Ustilago hordei, was previously shown to be genetically associated with a 50-kb deletion within a 940 -kb chromosome. Physiological studies suggested that a gene that funct ions upstream of adenylyl cyclase was deleted in the mutant. Represent ational difference analysis of isolated chromosomes was used to obtain deletion-specific DNA probes and corresponding genomic cosmid clones. Complementation analysis identified a cosmid clone and subsequently a 2.1-kb insert that converted transformants of the mutant strain10.1a( fill) from the filamentous to the sporidial cell type. A single open r eading frame of 354 codons that encodes a putative a-subunit of the he terotrimeric G-proteins was identified. Fill displayed a high degree o f sequence identity to Gpa1 from the basidiomycete Cryptococcus neofor mans and CPG-2 from the ascomycete Cryphonectria parasitica. FIL1, whe n introduced on a self-replicating vector, was found to suppress filam entous growth of starved haploid wild-type strains and restore normal mating response to the fill mutant, but did not suppress sexual dimorp hism of either strain. Fill appears to function analogously to mammali an G alpha proteins, which are coupled to cAMP production via adenylyl cyclase, to regulate dimorphic switching in U. hordei.