Figure 4.

Complementing and interfering phenotypes of the secY mutations. (A) Complementation of the secY39 protein export defect. Strain MR1 (secY39) was transformed with a SecY mutant plasmid indicated at the bottom. Cells were grown with glycerol and maltose as carbon sources, first at 37°C and then at 20°C for 30 min, and pulse-labeled with [³⁵S]methionine for 1 min (7). Labeled MBP was immunoprecipitated and proportions of radioactivities associated with the mature (presumably exported) form are quantitated by a PhosphorImager (7). The broken line shows the noncomplemented value. (B) Dominant interference. Strain AD202 (secY⁺) was transformed with two plasmids, pSTD343 (lacI^q; Y. Akiyama, personal communication) and one of the dominant-negative SecY plasmids as specified at the bottom. Cells were grown at 37°C in the presence of 1 mM isopropyl-thio-β-D-thiogalactoside and 5 mM cAMP and pulse-labeled with [³⁵S]methionine for 30 sec. The extents of MBP processing are shown as in A. The broken line shows the noninterfered value.