The highly reduced spliceosome of Cyanidioschyzon merolae

C. merolae is an extremophilic red alga, growing in volcanic hot springs at temperatures up to 56 C and pH as low as 0.3. The 2004 genome sequence revealed a surprising paucity of introns - only 27 for ~5500 genes. In 2015, we reported an initial characterization of the C. merolae spliceosome, which revealed the absence of the U1 snRNA, any U1 proteins, and the Prp28 ATPase that would normally mediate the switch from U1 to U6 at the 5’ splice site. In addition, the complement of splicing proteins appears to have been reduced to ~45, with notable absences in step-specific factors, assembly factors, and most of the NineTeen Complex. We have subsequently reported that, unlike other organisms, C. merolae has but a single LSm complex that is present in both the nucleus and the cytoplasm. Intriguingly, it appears that the splicing machinery is stably associated with the Pat1 degradation machinery that is normally found in the cytoplasm. We are currently addressing a number of questions with this unusual splicing system, which we believe represents a spliceosome stripped down to its catalytic essentials with little or no modulatory proteins.