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Vol. 8, Issue 10, 1989-2002, October 1997
Department of Biology, Center for Molecular Genetics,
University of California, San Diego, La Jolla, California 92093-0634
We have identified a developmentally essential gene,
UbcB, by insertional mutagenesis. The encoded protein
(UBC1) shows very high amino acid sequence identity to
ubiquitin-conjugating enzymes from other organisms, suggesting that
UBC1 is involved in protein ubiquitination and possibly degradation
during Dictyostelium development. Consistent with the
homology of the UBC1 protein to UBCs, the developmental pattern of
protein ubiquitination is altered in ubcB-null cells.
ubcB-null cells are blocked in the ability to properly
execute the developmental transition that occurs between the induction
of postaggregative gene expression during mound formation and the
induction of cell-type differentiation and subsequent morphogenesis.
ubcB-null cells plated on agar form mounds with normal
kinetics; however, they remain at this stage for ~10 h before forming
multiple tips and fingers that then arrest. Under other conditions,
some of the fingers form migrating slugs, but no culmination is
observed. In ubcB-null cells, postaggregative gene
transcripts accumulate to very high levels and do not decrease significantly with time as they do in wild-type cells. Expression of
cell-type-specific genes is very delayed, with the level of prespore-specific gene expression being significantly reduced compared
with that in wild-type cells. lacZ reporter studies
using developmentally regulated and cell-type-specific promoters
suggest that ubcB-null cells show an unusually elevated
level of staining of lacZ reporters expressed in
anterior-like cells, a regulatory cell population found scattered
throughout the aggregate, and reduced staining of a prespore reporter.
ubcB-null cells in a chimeric organism containing
predominantly wild-type cells are able to undergo terminal
differentiation but show altered spatial localization. In contrast, in
chimeras containing only a small fraction of wild-type cells, the
mature fruiting body is very small and composed almost exclusively of
wild-type cells, with the ubcB-null cells being present
as a mass of cells located in extreme posterior of the developing
organism. The amino acid sequence analysis of the UbcB
open reading frame (ORF) and the analysis of the developmental
phenotypes suggest that tip formation and subsequent development
requires specific protein ubiquitination, and possibly degradation.
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