EITHER DELTA-TUBULIN ISOGENE PRODUCT IS SUFFICIENT FOR MICROTUBULE FUNCTION DURING ALL STAGES OF GROWTH AND DIFFERENTIATION IN ASPERGILLUS-NIDULANS

The filamentous fungus AsPergillus nidulans has two genes encoding alp ha-tubulin, tubA and tubB, which are differentially required at distin ct stages during the life cycle. The tubA gene is required during vege tative growth for mitosis and nuclear migration (B. R. Oakley, C. E. O akley, and J. E. Rinehart, Mol. Gen. Genet. 208:135-144, 1987; P. Dosh i, C. A. Bossie, J. H. Doonan, G. S. May, and N. R. Morris, Mol. Gen. Genet. 225:129-141, 1991). The tubB gene is not required for any detec table aspect of vegetative growth or asexual reproduction but is essen tial during sexual development prior to the first meiotic division (K. E. Kirk and N. R. Morris, Genes Dev. 5:2014-2023, 1991). In this stud y, we determined whether the role of each alpha-tubulin gene is to pro vide a specific isotype necessary for a particular microtubule functio n or whether either alpha-tubulin isotype, if present in sufficient qu antities, can participate effectively in all types of microtubule. Str ains carrying a deletion allele of tubB (tubBDELTA) produce no ascospo res from a cross. When one copy of a plasmid containing the region ups tream of the tubB gene fused to the tubA coding region was integrated into a tubBDELTA strain, ascosporogenesis proceeded beyond the tubBDEL TA block and resulted in the formation of sexual spores. However, irre gular numbers of spores formed in some asci during development, and th e ascospores had greatly diminished viability and aberrant morphologie s. These defects were nearly corrected when two additional copies of t he tubA coding region were integrated into the tubBDELTA strain. These results indicate that the tubA alpha-tubulin isotype can form functio nal microtubules during sexual development in the absence of tubB prot ein. In a reciprocal set of experiments, we examined whether upregulat ion of tubB can complement the tubA4 mutation, which causes supersensi tivity to benomyl during vegetative growth. When tubA4 strains integra ted a plasmid containing an alcohol-inducible promoter joined to the t ubB coding region and subsequently overexpressed the tubB isotype, the benomyl supersensitivity normally caused by the tubA4 allele was reli eved. These results indicate that when enough tubB alpha-tubulin is su pplied, strains lacking functional tubA isotype can still form microtu bules which effectively carry out mitosis and nuclear migration.