Internal in-frame translation generates Cas11b, which is important for effective interference in an archaeal CRISPR-Cas system

Abstract

CRISPR-Cas is a sophisticated defence system used by bacteria and archaea to fend off invaders. CRISPR-Cas systems vary in their Cas protein composition and have therefore been divided into different classes and types. Type I systems of bacteria have been shown to contain the small Cas11 protein as part of the interference complex. Here we show for the first time that an archaeal CRISPR-Cas type I system also contains a Cas11 protein. In addition, we show for the first time an internal in-frame translation of an archaeal protein. The Cas11b protein from the Haloferax volcanii type I-B system is encoded in the cas8b gene. Translation initiation at an internal methionine of the cas8b open reading frame results in synthesis of Cas11b. Cas11b is required for an effective interference reaction and without Cas11b fewer Cascade complexes form. Comparison of transcriptomes from wild type and a Cas11b less strain show that the depletion of Cas11b results in differential regulation of many genes. Taken together Cas11b is important for the defence reaction of the type I-B CRISPR-Cas system and seems to play an additional cellular role.