S. cerevisiae selectively
uses good nitrogen sources (GLN) in preference to poor ones (PRO) by
repressing GATA-factor-dependent transcription of the genes needed to
transport and catabolize poor nitrogen sources, a physiological
process designated nitrogen catabolite repression (NCR). We show that
some NCR-sensitive genes produce two transcripts of slightly different
sizes. Synthesis of the shorter transcript is NCR-sensitive, while
that of the longer one is not. The longer transcript also predominates
in gln3 deletion mutants irrespective of the nitrogen source
provided. The longer mRNA species arises through the use of an
alternative transcription start site generated by the Gln3p binding
sites (GATAAs) acting as surrogate TATA elements. The ability of
GATAAs to serve as surrogate TATAs, i.e., when synthesis of the
shorter, NCR-sensitive transcript is inhibited, correlates with
sequestration of EGFP-Gln3p in the cytoplasm in a way that is
indistinguishable from that seen with EGFP-Ure2p. However, when the
shorter, NCR-sensitive transcript predominates, EGFP-Gln3p is
nuclear. From these data and a previous report, suggesting existence
of a Ure2p-Gln3p complex, we conclude that formation of a cytoplasmic
Gln3p-Ure2p complex results in Gln3p being excluded from the nucleus.
This in turn prevents Gln3p from reaching its GATAA binding sites
upstream of NCR-sensitive genes and activating their transcription.
Supported by NIH grant GM-35642.
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