Supermajority of Chlamydia trachomatis genes are activated during the first hour of infection
The obligate intracellular bacterium Chlamydia has a unique developmental cycle that alternates between two contrasting cell types. With a hardy envelope and highly condensed genome, the small elementary body (EB) maintains limited metabolic activities yet can survive in an extracellular environment and is infectious. After entering host cells, EBs differentiate into larger and proliferating reticulate bodies (RBs). Progeny EBs are derived from RBs in late developmental stages and eventually exit host cells. How the expression of the chlamydial genome consisting of nearly 1000 genes governs the chlamydial developmental cycle is unclear. A previous microarray study identified only 29 immediately early genes, defined as genes activated by the first hour postinoculation, in C. trachomatis. By performing RNA sequencing analysis for C. trachomatis cultures with high multiplicities of infection (i.e., MOI of 50 and 200), we observed that 730 C. trachomatis genes underwent 2- to 900-fold activation within one hour postinoculation. By conducting quantitative reverse transcription real-time PCR (qRT-PCR) analysis for 48 of the 730 genes using an MOI of 1, we confirmed the expression increases in 46 genes. Our results demonstrate that the immediate early transcriptome is tens of times more extensive than previously realized. Gene ontology analysis indicates that the activation spans across all functional categories. We conclude that a supermajority of the C. trachomatis genes are activated almost immediately after EBs are inside host cells to initiate the differentiation toward RBs and to establish an intracellular niche conducive for chlamydial development and growth. RNA-Seq analysis was performed for Chlamydia trachomatis L2 Overall design: Total RNA was prepared at 0 and 1 hpi.