The peptide mating pheromone a-factor and the hydrolytic enzyme invertase (f3D-fructofuranoside fructohydrolase, EC 3.2.1.26) are processed from larger precursor proteins during their secretion from yeast cells (Saccharomyces cerevisiae). An in-frame fusion of the structural genes for these two proteins was constructed by connecting the 5'-flanking region and preproleader portion of the coding sequence of the a-factor gene (MFa1l) to a large fragment of the invertase gene (SUC2) lacking its 5'-flanking region and the coding information for the first four amino acids of its signal sequence. Sites that have been implicated in normal proteolytic processing of the a-factor precursor have been retained in this construction. The chimeric gene directs synthesis of a high level of active invertase that is secreted efficiently into the periplasmic space, permitting cell growth on sucrose-containing media. This extracellular invertase appears to contain no prepro-a-factor sequences. The initial intracellular product is, however, a hybrid protein that can be detected either by treatment of the cells with the drug tunicamycin or by blockage of secretion in a temperature-conditional secretion-defective mutant (secl8). Therefore, prior to its efficient proteolytic removal, the a-factor portion of the hybrid protein apparently provides the necessary information for efficient export of the substantially larger protein invertase. Similar to MFal, the MFal-SUC2 fusion is expressed in a haploids at levels 65-75 times higher than in a haploids or in a/a diploids; also, high-level expression is eliminated in matal mutants but not in matat mutants. Unlike expression of SUC2, expression of the fusion is not affected by glucose concentration. Hence, the 5'-flanking region present in the fusion (about 950 base pairs) is sufficient to confer a cell-specific expression to the hybrid gene.
Data from the California Prenatal Screening Program indicate that all of the major screening modalities continue to be utilized. The wide range of choices made by women with screen positive results demonstrate the importance of including multiple options within the Program. Providing integrated screening to first trimester Down syndrome screen positive women reduced the number of unnecessary invasive procedures.
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