Several yeast genes produce multiple transcripts with different 3'-ends. Of these, four genes are known to produce truncated transcripts that end within the coding sequence of longer transcripts: CBP1 , AEP2 / ATP13 , RNA14 and SIR1 . It has been shown that the level of the truncated CBP1 transcript increases during the switch to respiratory growth while that of the full-length transcript decreases. To determine whether this phenomenon is unique to CBP1 , northern analysis was used to determine whether the levels of other truncated transcripts are regulated similarly by carbon source. The levels of the shortest transcripts of AEP2 / ATP13 and RNA14 increased during respiration while the shortest SIR1 transcript remained constant. However, two longer SIR1 transcripts were regulated reciprocally by carbon source. Mapping the 3'-ends of each transcript by sequencing partial cDNA clones revealed multiple 3'-ends for each transcript. Examination of the sequences surrounding the 3'-ends of the induced transcripts failed to identify a consensus sequence but did reveal weak putative 3'-end formation signals in all of the transcripts. Similarly, no consensus sequence was found when the sequences surrounding the 3'-ends of the longest transcripts were compared, but again weak putative 3'-end formation signals were identified. These data are suggestive of carbon source regulation of alternative poly(A) site choice in yeast.

The yeast mitochondrial genome encodes only seven major components of the respiratory chain and ATP synthase; more than 200 other mitochondrial proteins are encoded by nuclear genes. Thus, assembly of functional mitochondria requires coordinate expression of nuclear and mitochondrial genes. One example of coordinate regulation is the stabilization of mitochondrial COB (cytochrome b) mRNA by Cbp1, the product of the nuclear gene CBP1 (cytochrome b processing). CBP1 produces two types of transcripts with different 3 ends: full-length 2.2-kb transcripts and 1.2-kb transcripts truncated within the coding sequence of Cbp1. Upon induction of respiration, the steady-state level of the long transcripts decreases while that of the short transcripts increases reciprocally, an unexpected result since the product of the long transcripts is required for COB mRNA stability and thus for respiration. Here we have tested the hypothesis that the short transcripts, or proteins translated from the short transcripts, are also required for respiration. A protein translated from the short transcripts was not detected by Western analysis, although polysome gradient fractions were shown to contain both long and short CBP1 transcripts. A mutant strain in which production of the short transcripts was abolished showed wild-type growth properties, indicating that the short transcripts are not required for respiration. Due to mutation of the carbon source-responsive element, the long transcript level in the mutant strain did not decrease during induction of respiration. The mutant strain had increased levels of COB RNA, suggestive that production of short CBP1 transcripts is a mechanism for downregulation of the levels of long CBP1 transcripts, Cbp1, and COB mRNA during the induction of respiration.The yeast Saccharomyces cerevisiae is a facultative organism; it can obtain energy by either respiration or fermentation (for reviews, see references 45 and 56). Since respiration is not essential, respiratory-deficient mutant strains can be isolated and maintained on a fermentable carbon source such as glucose. Nuclear mutations describe more than 200 genes required for mitochondrial function (for a review, see reference 56). A subset of the nuclear gene products interacts with mitochondrial mRNAs to promote stability, processing, and translation (for a review, see reference 13). One example of such an interaction is that between Cbp1 and COB (cytochrome b) RNA. Cbp1 is encoded by the nuclear gene CBP1 (cytochrome b processing) and is imported into mitochondria, where it is necessary for the production of stable COB mRNA (6,12,36,49,58).The CBP1 gene produces two types of transcripts that differ at the 3Ј end ( Fig. 1 and reference 32). The full-length transcripts are 2.2 kb in length; the shorter transcripts are 1.2 kb in length and have 3Ј ends within the coding sequence of Cbp1. Production of the 2.2-and 1.2-kb transcripts is differentially regulated by carbon source (32, 33). When cells are switched from a fermentable carbon source (e.g., glu...

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Suspended sentences, a form of leniency, are used by judges for deserving offenders. There are two kinds of suspended sentence, that is, suspended imposition of sentence and suspended execution of sentence. Suspended sentences were granted by judges centuries before the advent of probation; even so, they are the foundation for probation. More suspended sentences are granted by courts each year than any other sentence; however, most suspended sentences do not require probation.