Which organism listed below lacks a classical linker histone H1 (has no canonical H1, only a divergent or absent equivalent)?

Introduction / Context:
Histone H1 is a linker histone that binds at the DNA entry/exit points of the nucleosome and helps higher-order chromatin folding. Some organisms do not carry a canonical H1 ortholog.

Given Data / Assumptions:

• Classical H1 is abundant in many metazoans and plants.
• Yeast chromatin is compacted primarily through core histones and other architectural factors.

Concept / Approach:
Identify species known to lack a canonical H1. Budding yeast Saccharomyces cerevisiae lacks a typical H1; it possesses Hho1p, a divergent protein that does not function as a standard linker histone across the genome.

Step-by-Step Solution:
1) Consider model organisms: Xenopus and sea urchin nuclei are rich in H1 variants; Arabidopsis encodes several H1 isoforms.2) Budding yeast lacks a classical H1 and relies on alternative mechanisms for chromatin organization.

Verification / Alternative check:
Genomic and proteomic surveys show absence of a canonical H1 gene in S. cerevisiae and only a divergent Hho1 protein with limited linker-like roles.

Why Other Options Are Wrong:
Sea urchin, frog, and Arabidopsis have well-characterized H1 proteins and variants participating in chromatin compaction.

Common Pitfalls:
Assuming all eukaryotes share identical histone complements; fungi often deviate from metazoan norms.

Final Answer:
Saccharomyces cerevisiae