KRB1, a suppressor of mak7-1 (a mutant RPL4A), is RPL4B, a second ribosomal protein L4 gene, on a fragment of Saccharomyces chromosome XII.

The mak7-1 mutant loses the killer toxin-encoding M1 dsRNA. MAK7 is RPL4A, one of two genes encoding ribosomal protein L4. KRB1 is a dominant suppressor of mak7-1 that is tightly centromerelinked, but not linked to centromere markers of chromosomes I-XVI. Our orthogonal field agarose gel electrophoresis analysis of chromosomal DNA from strains with KRB1 shows a novel band of approximately 250 kb. This band hybridizes with an RPL4B-specific probe, but not an RPL4A (MAK7)-specific probe. The RPL4B-specific probe also hybridizes to chromosome XII where the original RPL4B is located. KRB1 is meiotically linked to this extra chromosome. Disruption of either the RPL4B gene on chromosome XII or that on the extra chromosome results in loss of the killer phenotype and a decreased concentration of free 60S subunits. Thus, the RPL4B on the extra chromosome is KRB1 and is active. The extra chromosome contains chromosome XII sequence between Lambda 5345 clone (ATCC70558) and Lambda 6639 clone (ATCC71085) of Olson's Lambda library, indicating that KRB1 represents a chromosomal rearrangement involving chromosome XII and explaining the earlier genetic data.